fs: Add new VFCF_xxx flags for va_filerev

Richard Kojedzinszky <richard@kojedz.in> reported a problem via
email, where the Linux NFSv4.2 client did not detect a change in a
directory on a FreeBSD NFSv4.2 server.

Adding support for the NFSv4.2 change_attr_type attribute seems
to have fixed the problem. This requires that the server file system
indicate if it increments va_filerev by one, since that file attribute
is used for the NFSv4.2 change attribute. Fuse requires an indication
that va_filerev is based on ctime.

This patch adds VFCF_FILEREVINC and VFCF_FILEREVCT to indicate this.

A future patch to the NFS server will use these flags.

Reviewed by: asomers, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D48436