erofs: use separate xattr parsers for listxattr/getxattr

There's a callback styled xattr parser, i.e. xattr_foreach(), which is
shared among listxattr and getxattr.

Convert it to two separate xattr parsers to serve listxattr and getxattr
for better readability.
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230613074114.120115-6-jefflexu@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>