block: Add bio_max_segs

It's often inconvenient to use BIO_MAX_PAGES due to min() requiring the
sign to be the same.  Introduce bio_max_segs() and change BIO_MAX_PAGES to
be unsigned to make it easier for the users.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index f014c5e4..3db9234 100644
--- a/fs/ext4/readpage.c
+++ b/fs/ext4/readpage.c
@@ -371,8 +371,7 @@ int ext4_mpage_readpages(struct inode *inode,
 			 * bio_alloc will _always_ be able to allocate a bio if
 			 * __GFP_DIRECT_RECLAIM is set, see bio_alloc_bioset().
 			 */
-			bio = bio_alloc(GFP_KERNEL,
-				min_t(int, nr_pages, BIO_MAX_PAGES));
+			bio = bio_alloc(GFP_KERNEL, bio_max_segs(nr_pages));
 			fscrypt_set_bio_crypt_ctx(bio, inode, next_block,
 						  GFP_KERNEL);
 			ext4_set_bio_post_read_ctx(bio, inode, page->index);