geom_label: Add more validation for NTFS volume tasting

geom_label: Add more validation for NTFS volume tasting

- Ensure that the computed MFT record size isn't negative or larger than
  maxphys before trying to read $Volume.
- Guard against truncated records in volume metadata.
- Ensure that the record length is large enough to contain the volume
  name.
- Verify that the (UTF-16-encoded) volume name's length is a multiple of
  two.

PR:		258833, 258914
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation