It generalizes the VFS_FHTOVP() interface, making it possible to fetch
the inode without faking filehandle.  Also it adds the ffs flags argument
which allows to control ffs_vgetf() call.

Requested by:	mckusick
Reviewed by:	chs, mckusick
Tested by:	pho
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation