UPSTREAM: exfat: short-circuit zero-byte writes in exfat_file_write_iter

When generic_write_checks() returns zero, it means that
iov_iter_count() is zero, and there is no work to do.

Simply return success like all other filesystems do, rather than
proceeding down the write path, which today yields an -EFAULT in
generic_perform_write() via the
(fault_in_iov_iter_readable(i, bytes) == bytes) check when bytes
== 0.

Fixes: 11a347fb ("exfat: change to get file size from DataLength")
Reported-by: Noah <kernel-org-10@maxgrass.eu>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
(cherry picked from commit fda94a99)
Change-Id: I5686d6d1029a6563d5a45541735e6de6291b3645
Bug: 397325648
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Lee Jones <joneslee@google.com>