smb: client: Fix -Wstringop-overflow issues

pSMB->hdr.Protocol is an array of size 4 bytes, hence when the compiler
analyzes this line of code

	parm_data = ((char *) &pSMB->hdr.Protocol) + offset;

it legitimately complains about the fact that offset points outside the
bounds of the array. Notice that the compiler gives priority to the object
as an array, rather than merely the address of one more byte in a structure
to wich offset should be added (which seems to be the actual intention of
the original implementation).

Fix this by explicitly instructing the compiler to treat the code as a
sequence of bytes in struct smb_com_transaction2_spi_req, and not as an
array accessed through pointer notation.

Notice that ((char *)pSMB) + sizeof(pSMB->hdr.smb_buf_length) points to
the same address as ((char *) &pSMB->hdr.Protocol), therefore this results
in no differences in binary output.

Fixes the following -Wstringop-overflow warnings when built s390
architecture with defconfig (GCC 13):
  CC [M]  fs/smb/client/cifssmb.o
In function 'cifs_init_ace',
    inlined from 'posix_acl_to_cifs' at fs/smb/client/cifssmb.c:3046:3,
    inlined from 'cifs_do_set_acl' at fs/smb/client/cifssmb.c:3191:15:
fs/smb/client/cifssmb.c:2987:31: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
 2987 |         cifs_ace->cifs_e_perm = local_ace->e_perm;
      |         ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
In file included from fs/smb/client/cifssmb.c:27:
fs/smb/client/cifspdu.h: In function 'cifs_do_set_acl':
fs/smb/client/cifspdu.h:384:14: note: at offset [7, 11] into destination object 'Protocol' of size 4
  384 |         __u8 Protocol[4];
      |              ^~~~~~~~
In function 'cifs_init_ace',
    inlined from 'posix_acl_to_cifs' at fs/smb/client/cifssmb.c:3046:3,
    inlined from 'cifs_do_set_acl' at fs/smb/client/cifssmb.c:3191:15:
fs/smb/client/cifssmb.c:2988:30: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
 2988 |         cifs_ace->cifs_e_tag =  local_ace->e_tag;
      |         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
fs/smb/client/cifspdu.h: In function 'cifs_do_set_acl':
fs/smb/client/cifspdu.h:384:14: note: at offset [6, 10] into destination object 'Protocol' of size 4
  384 |         __u8 Protocol[4];
      |              ^~~~~~~~

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: KSPP#310
Fixes: dc1af4c ("cifs: implement set acl method")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: GustavoARSilva

fs/smb/client/cifssmb.c

@@ -3184,7 +3184,7 @@ int cifs_do_set_acl(const unsigned int xid, struct cifs_tcon *tcon,
 	param_offset = offsetof(struct smb_com_transaction2_spi_req,
 				InformationLevel) - 4;
 	offset = param_offset + params;
-	parm_data = ((char *) &pSMB->hdr.Protocol) + offset;
+	parm_data = ((char *)pSMB) + sizeof(pSMB->hdr.smb_buf_length) + offset;
 	pSMB->ParameterOffset = cpu_to_le16(param_offset);
 
 	/* convert to on the wire format for POSIX ACL */