fs/direct-io.c: don't try to allocate more than BIO_MAX_PAGES in a bio

When using devices that support max_segments > BIO_MAX_PAGES (256),
direct IO tries to allocate a bio with more pages than allowed, which
leads to an oops in dio_bio_alloc(). Clamp that request to the supported
maximum.

Signed-off-by: David Dillow <redacted>
--
dio_bio_alloc() doesn't check the result of bio_alloc(), so it
dereferences a NULL pointer. bio_alloc(GFP_KERNEL, ...) doesn't fail
unless it gets called for an invalid number of pages, so it seems a bit
like overkill to check for failure in dio_bio_alloc(), though it would
have saved me some time tracking this down.

diff --git a/fs/direct-io.c b/fs/direct-io.c
index 85882f6..9eb0553 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c

@@ -583,6 +583,7 @@ static int dio_new_bio(struct dio *dio, sector_t start_sector)
 		goto out;
 	sector = start_sector << (dio->blkbits - 9);
 	nr_pages = min(dio->pages_in_io, bio_get_nr_vecs(dio->map_bh.b_bdev));
+	nr_pages = min(nr_pages, BIO_MAX_PAGES);
 	BUG_ON(nr_pages <= 0);
 	ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
 	dio->boundary = 0;