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

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.

I have done the same thing myself, commonly when using scrub, but also using add/del device. It's most alarming!

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