FreeBSD: Add setting of the VFCF_FILEREV flag

The flag VFCF_FILEREV was recently defined in FreeBSD
so that a file system could indicate that it increments
va_filerev by one for each change.

Since ZFS does do this, set the flag if defined for the
kernel being built. This allows the NFSv4.2 server to
reply with the correct change_attr_type attribute value.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Closed #16976