Change the semantics of i_modrev/va_filerev to what is required for
	the nfsv4 Change attribute. There are 2 changes:
	1 - The value now changes on metadata changes as well as data
	    modifications (incremented for IN_CHANGE instead of IN_UPDATE).
	2 - It is now saved in spare space in the on-disk i-node so that it
	    survives a crash.
	Since va_filerev is not passed out into user space, the only current
	use of va_filerev is in the nfs server, which uses it as the directory
	cookie verifier. Since this verifier is only passed back to the server
	by a client verbatim and then the server doesn't check it, changing the
	semantics should not break anything currently in FreeBSD.