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