BUG :: btrfs resize should require mount point not just /some/path

[Robert White <rwhite@pobox.com>]

So here's a thing...

If you've got a BTRFS root file system and you mount go to resize a 
removable media and you make a typo you can easily resize your root 
instead of a target.

mkdir /media/vol1 /media/vol2
mount /dev/sdz1 /media/vol2 # intended vol1
btrfs resize -32G /media/vol1
umount /dev/sdz1
#proceed to fark up /dev/sdz1 with partitioning tools

Since resize will accept _any_ existing directory (or even filename?) as 
the target of a resize instead of limiting the targets to known 
mount-points it pretty much invites mistakes.

The system _should_ error out on any path thats not a mount point in the 
name of safety alone.

A message like "/some/path not a mounted file system" would be less 
surprising than operating on a file system when any arbitrary directory 
is named.