zfs-functions: skip lines where mntpnt is 'none'

zfs-functions: skip lines where mntpnt is 'none'

This fixes zfs-mount initscript trying to mount swap volumes
as filesystems or anything that has 'none' as a mountpoint
in /etc/fstab.  Additionally, fixes trying to mount swap volumes
as a filesystem on RHEL.  RHEL defines mountpoint for swap
as `swap`.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Georgy Yakovlev <ya@sysdump.net>
Closes #7346
Closes #7347