ovl: fold ovl_getxattr() into ovl_get_redirect_xattr()

This is a partial revert (with some cleanups) of commit 993a0b2a ("ovl:
Do not lose security.capability xattr over metadata file copy-up"), which
introduced ovl_getxattr() in the first place.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>