erofs: shorten bvecs[] for file-backed mounts

BIO_MAX_VECS is too large for __GFP_NOFAIL allocation.  We could use
a mempool (since BIOs can always proceed), but it seems overly
complicated for now.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250107082825.74242-1-hsiangkao@linux.alibaba.com

fs/erofs/fileio.c

@@ -6,7 +6,7 @@
 #include <trace/events/erofs.h>
 
 struct erofs_fileio_rq {
-	struct bio_vec bvecs[BIO_MAX_VECS];
+	struct bio_vec bvecs[16];
 	struct bio bio;
 	struct kiocb iocb;
 	struct super_block *sb;
@@ -68,7 +68,7 @@ static struct erofs_fileio_rq *erofs_fileio_rq_alloc(struct erofs_map_dev *mdev)
 	struct erofs_fileio_rq *rq = kzalloc(sizeof(*rq),
 					     GFP_KERNEL | __GFP_NOFAIL);
 
-	bio_init(&rq->bio, NULL, rq->bvecs, BIO_MAX_VECS, REQ_OP_READ);
+	bio_init(&rq->bio, NULL, rq->bvecs, ARRAY_SIZE(rq->bvecs), REQ_OP_READ);
 	rq->iocb.ki_filp = mdev->m_dif->file;
 	rq->sb = mdev->m_sb;
 	return rq;