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 <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Gao Xiang <[email protected]>

Author: lostjeffle