dm mpath: free path selector on invalid args

Free path selector if the arguments are invalid.

This command (note that it is invalid) causes reference leak on module
"dm_round_robin" and prevents the module from being removed.

dmsetup create --table '0 2 multipath 0 0 1 1 round-robin /dev/sdh' mpath0

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>