staging: lustre: acl: Remove lustre_posix_acl_xattr_free wrapper

Remove the wrapper function lustre_posix_acl_xattr_free() and replace its
call in the file xattr with the function kfree() that it wrapped. Also,
its prototype from the header lustre_eacl is removed as it is no longer
of any use.

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>