libafs: afs_CacheFetchProc can't be called without a dcache pointer

libafs: afs_CacheFetchProc can't be called without a dcache pointer

An inspection of the only call site suggests that afs_CacheFetchProc()
can't be called with a null dcache pointer, and code further down
in this function dereferences adc unconditionally (assuming
rxfs_fetchInit() doesn't crash first) so remove the conditional
here.

Probably more of these parameters can and should be included in the
AFS_NONNULL.

Change-Id: Ic87517376085b0d5bc7631b5558411259ae986f4
Reviewed-on: http://gerrit.openafs.org/5180
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>