mm/filemap.c: Use existing variable

Use the variable write_len instead of ov_iter_count(from).

Link: http://lkml.kernel.org/r/1537375855-2088-1-git-send-email-leviathan0992@gmail.comSigned-off-by: Nhaiqing.shq <leviathan0992@gmail.com>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -3027,7 +3027,7 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	if (iocb->ki_flags & IOCB_NOWAIT) {
 		/* If there are pages to writeback, return */
 		if (filemap_range_has_page(inode->i_mapping, pos,
-					   pos + iov_iter_count(from)))
+					   pos + write_len))
 			return -EAGAIN;
 	} else {
 		written = filemap_write_and_wait_range(mapping, pos,