cifs: track writepages in vfs operation counters

writepages and readpages operations did not call get/free_xid
so the statistics for file copy could get confusing with "vfs operations"
not increasing.  Add get_xid and free_xid to cifs readpages and
writepages functions.

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>