cifs: don't check for failure from mempool_alloc()

mempool_alloc() cannot fail if the gfp flags allow it to
sleep, and both GFP_FS allows for sleeping.

So these tests of the return value from mempool_alloc()
cannot be needed.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Steve French <smfrench@gmail.com>