LU-7002 utils: SELinux context repeated in mount opts

LU-7002 utils: SELinux context repeated in mount opts

SELinux context can only be specified once in mount options,
thus append_context_for_mount() should not apply once again
when called at mount time.

Signed-off-by: Frederic Saunier <frederic.saunier@atos.net>
Change-Id: I350b67ec42691b875aab085224185f2a7582d41d
Reviewed-on: http://review.whamcloud.com/18319
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Grégoire Pichon <gregoire.pichon@bull.net>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>