erofs: introduce xattrs replacement to C

This patch introduces erofs_getxattr_rust and erofs_listxattr_rust to C
and can replace the original xattr logic entirely.

Note that the original acl implementation is tweaked with a lifted
function called erofs_getxattr_nobuf, so that difference of the calling
convention of Rust side code can be bridged.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>