nfsd4: factor ctime into change attribute

Factoring ctime into the nfsv4 change attribute gives us better
properties than just i_version alone.

Eventually we'll likely also expose this (as opposed to raw i_version)
to userspace, at which point we'll want to move it to a common helper,
called from either userspace or individual filesystems.  For now, nfsd
is the only user.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>