ext4: remove the null check of bio_vec page

bv_page can't be NULL in a valid bio_vec, so we can remove the NULL check,
as we did in other places when calling bio_for_each_segment_all() to go
through all bio_vec of a bio.
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
Link: https://lore.kernel.org/r/20201020082201.34257-1-tian.xianting@h3c.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/page-io.c

@@ -111,9 +111,6 @@ static void ext4_finish_bio(struct bio *bio)
 		unsigned under_io = 0;
 		unsigned long flags;
 
-		if (!page)
-			continue;
-
 		if (fscrypt_is_bounce_page(page)) {
 			bounce_page = page;
 			page = fscrypt_pagecache_page(bounce_page);