fsdax: Fix infinite loop in dax_iomap_rw()

I got an infinite loop and a WARNING report when executing a tail command
in virtiofs.

  WARNING: CPU: 10 PID: 964 at fs/iomap/iter.c:34 iomap_iter+0x3a2/0x3d0
  Modules linked in:
  CPU: 10 PID: 964 Comm: tail Not tainted 5.19.0-rc7
  Call Trace:
  <TASK>
  dax_iomap_rw+0xea/0x620
  ? __this_cpu_preempt_check+0x13/0x20
  fuse_dax_read_iter+0x47/0x80
  fuse_file_read_iter+0xae/0xd0
  new_sync_read+0xfe/0x180
  ? 0xffffffff81000000
  vfs_read+0x14d/0x1a0
  ksys_read+0x6d/0xf0
  __x64_sys_read+0x1a/0x20
  do_syscall_64+0x3b/0x90
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

The tail command will call read() with a count of 0. In this case,
iomap_iter() will report this WARNING, and always return 1 which casuing
the infinite loop in dax_iomap_rw().

Fixing by checking count whether is 0 in dax_iomap_rw().

Fixes: ca289e0 ("fsdax: switch dax_iomap_rw to use iomap_iter")
Signed-off-by: Li Jinlin <lijinlin3@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20220725032050.3873372-1-lijinlin3@huawei.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: Li Jinlin

fs/dax.c

@@ -1241,6 +1241,9 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
 	loff_t done = 0;
 	int ret;
 
+	if (!iomi.len)
+		return 0;
+
 	if (iov_iter_rw(iter) == WRITE) {
 		lockdep_assert_held_write(&iomi.inode->i_rwsem);
 		iomi.flags |= IOMAP_WRITE;