ext4: use pagevec_lookup_range() in ext4_find_unwritten_pgoff()

Use pagevec_lookup_range() in ext4_find_unwritten_pgoff() since we are
interested only in pages in the given range.  Simplify the logic as a
result of not getting pages out of range and index getting automatically
advanced.

Link: http://lkml.kernel.org/r/20170726114704.7626-6-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>