staging/lustre/mdc: Initialize req in mdc_enqueue for !it case

Commit ab909585b813 ("staging: lustre: Cleanup variable declarations
in mdc_enqueue()") broke Lustre flock handling introducing access
to uninitialized req variable, leading to bizzare crash in a later
call to __req_capsule_offset with invalid pill value.

Set req to NULL just for this case as in all other cases req is
explicitly initialized with request packing call.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
CC: Srikrishan Malik <srikrishanmalik@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>