filesystems that have block pointers that are out-of-range for their
filesystem. These out-of-range block pointers are corrected by
fsck(8) so are only encountered when an unchecked filesystem is
mounted.

A new "untrusted" flag has been added to the generic mount interface
that can be set when mounting media of unknown provenance or integrity.
For example, a daemon that automounts a filesystem on a flash drive
when it is plugged into a system.

This commit adds a test to UFS/FFS that validates all block numbers
before using them. Because checking for out-of-range blocks adds
unnecessary overhead to normal operation, the tests are only done
when the filesystem is mounted as an "untrusted" filesystem.

Reported by:  Christopher Krah, Thomas Barabosch, and Jan-Niclas Hilgert of Fraunhofer FKIE
Reported as:  FS-14-UFS-3: Out of bounds read in write-2 (ffs_alloccg)
Reviewed by:  kib
Sponsored by: Netflix