9pfs: fix filesystem to wait for stable page writeback

Fix up the ->page_mkwrite handler to provide stable page writes if necessary.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/9p/vfs_file.c

@@ -616,6 +616,7 @@ v9fs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	lock_page(page);
 	if (page->mapping != inode->i_mapping)
 		goto out_unlock;
+	wait_for_stable_page(page);
 
 	return VM_FAULT_LOCKED;
 out_unlock: