add fsfreeze helper:

add fsfreeze helper:

fsfreeze_mountpoint issues the same ioctls as fsfreeze(8) on the provided
directory (the $thaw parameter deciding between '--freeze' and '--unfreeze')

This is used for container backups on RBD, where snapshots on containers,
which are heavy on IO, are not mountable readonly, because the ext4 is not
consistent.

Needed to fix #2991 and #2528.

The ioctl numbers were found via strace -X verbose (and verified with the
kernel documentation).

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>