erofs: support SEEK_HOLE/SEEK_DATA in inode_share mode

When inode_share is enabled, erofs_ishare_fops.llseek falls back to
generic_file_llseek, which treats the whole file as data and always
returns i_size for SEEK_HOLE, hiding real holes in sparse files.

Switch it to erofs_file_llseek instead.  For user files f_mapping->host
is always the real erofs inode, so SEEK_HOLE/SEEK_DATA resolve the
per-file on-disk layout via iomap_seek_hole()/iomap_seek_data().

Reviewed-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <xiang@kernel.org>