Remount / with barrier=0 when fstype is ext4

Numerous people¹ have reported that `echo mem > /sys/power/state` fails
to suspend their machines. This is likely due to the following ext4
commit², which was created to increase data integrity:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=06a407f13daf9e48f0ef7189c7e54082b53940c7

kernel/power/suspend.c:pm_suspend() calls sys_sync() during sleep
preparation, which in turn calls fs/ext4/super.c:ext4_sync_fs(). After
the above commit, if the `sbi->s_journal->j_flags & JBD2_BARRIER`
condition returns >0, a final flush + wait is called on the underlying
block device.

Since the block device is the one we just suspended with `luksSuspend`,
the call hangs indefinitely and the machine never enters sleep.

Mounting an ext4 fs with the `barrier=0` or `nobarrier` option avoids
this lockup.

The corresponding ext3 sync fn does not appear to be affected, but this
is untested.

¹ @seanlynch in issue #2, a user on the AUR project page, and a Red Hat
  user in the following bug report:

  https://bugzilla.redhat.com/show_bug.cgi?id=1013304#c0


PATCH NOTES:

I changed the #! to /bin/bash to take advantage of the `local` statement
and the `${VAR:i:n}` expansion; I am happy to revert these back to POSIX
equivalents if /bin/sh compat is important to you.

Thanks!