umount.nfs: ignore non-nfs filesystems

 # umount.nfs /boot; echo $?
  umount.nfs: /boot: device is busy
  32

expected and fixed behavior:

 # umount.nfs /boot; echo $?
 # umount.nfs: /boot: is not an NFS filesystem
 1

Note that the function mnt_context_set_fstype_pattern() has never
been used for mtab/fstab evaluation. It's usable only for "umount -a"
and for "mount -t" operations.

Reported-by: NeilBrown <neilb@suse.de>
Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>