btrfs: accessors: compile-time fast path for u8

Reading/writing 1 byte (u8) is a special case compared to the others as
it's always contained in the folio we find, so the split memcpy will
never be needed. Turn it to a compile-time check that the memcpy part
can be optimized out.

The stack usage is reduced:

  btrfs_set_8                                         -16 (32 -> 16)
  btrfs_get_8                                         -16 (24 -> 8)

Code size reduction:

     text    data     bss     dec     hex filename
  1454951  115665   16088 1586704  183610 pre/btrfs.ko
  1454691  115665   16088 1586444  18350c post/btrfs.ko

  DELTA: -260

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

fs/btrfs/accessors.c

@@ -55,7 +55,8 @@ u##bits btrfs_get_##bits(const struct extent_buffer *eb,		\
 		report_setget_bounds(eb, ptr, off, sizeof(u##bits));	\
 		return 0;						\
 	}								\
-	if (INLINE_EXTENT_BUFFER_PAGES == 1 || likely(sizeof(u##bits) <= part))	\
+	if (INLINE_EXTENT_BUFFER_PAGES == 1 || sizeof(u##bits) == 1 ||	\
+	    likely(sizeof(u##bits) <= part))				\
 		return get_unaligned_le##bits(kaddr + oil);		\
 									\
 	memcpy(lebytes, kaddr + oil, part);				\
@@ -78,7 +79,7 @@ void btrfs_set_##bits(const struct extent_buffer *eb, void *ptr,	\
 		report_setget_bounds(eb, ptr, off, sizeof(u##bits));	\
 		return;							\
 	}								\
-	if (INLINE_EXTENT_BUFFER_PAGES == 1 ||				\
+	if (INLINE_EXTENT_BUFFER_PAGES == 1 || sizeof(u##bits) == 1 ||	\
 	    likely(sizeof(u##bits) <= part)) {				\
 		put_unaligned_le##bits(val, kaddr + oil);		\
 		return;							\