nfsd: only call inode_query_iversion in the I_VERSION case

inode_query_iversion() can modify i_version.  Depending on the exported
filesystem, that may not be safe.  For example, if you're re-exporting
NFS, NFS stores the server's change attribute in i_version and does not
expect it to be modified locally.  This has been observed causing
unnecessary cache invalidations.

The way a filesystem indicates that it's OK to call
inode_query_iverson() is by setting SB_I_VERSION.

So, move the I_VERSION check out of encode_change(), where it's used
only in GETATTR responses, to nfsd4_change_attribute(), which is
also called for pre- and post- operation attributes.

(Note we could also pull the NFSEXP_V4ROOT case into
nfsd4_change_attribute() as well.  That would actually be a no-op,
since pre/post attrs are only used for metadata-modifying operations,
and V4ROOT exports are read-only.  But we might make the change in
the future just for simplicity.)

Reported-by: Daire Byrne <daire@dneg.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>