cifs: remove superfluous inode_lock in cifs_{strict_}fsync

Originally, filemap_write_and_wait took the i_mutex internally, but
commit 02c24a82 pushed the mutex acquisition into the individual
fsync routines, leaving it up to the subsystem maintainers to remove
it if it wasn't needed.

For cifs, I see no reason to take the inode_lock here. All of the
operations inside that lock are protected in other ways.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>