erofs: fix blksize < PAGE_SIZE for file-backed mounts

erofs: fix blksize < PAGE_SIZE for file-backed mounts

Adjust sb->s_blocksize{,_bits} directly for file-backed
mounts when the fs block size is smaller than PAGE_SIZE.

Previously, EROFS used sb_set_blocksize(), which caused
a panic if bdev-backed mounts is not used.

Fixes: fb176750266a ("erofs: add file-backed mount support")
Signed-off-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241015103836.3757438-1-hongzhen@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>