libsmack: Removes checking smackfs isn't read only

Assuming that smack is available only if the filesystem
smackfs is mounted without being set to read-only have
negative side effects on tools like 'id', 'ls', 'ps'.
In effect, these tools are using libsmack to detect
availability of Smack and to tune their output to
print contexts.

This fixes smack-team/smack#105

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
Merge pull request #105 from jobol/issue105

libsmack: `verify_smackfs_mnt` wrongly expects smackfs to be writable