afs: Handle osi_NewVnode failures

afs: Handle osi_NewVnode failures

Currently, code inside afs_vcache.c assumes that osi_NewVnode always
returns non-NULL, which means that osi_NewVnode must panic if it
cannot create a new vnode.

All of the callers of afs_GetVCache, afs_NewVCache, etc, already
handle getting a NULL return, though (after all, the given fid may not
exist or be inaccessible due to network errors, etc). So, just
propagate NULL returns from osi_NewVnode up to our callers, to avoid
panics in these situations.

Modify osi_NewVnode on many arches to return an error on allocation
failure, instead of panic'ing.

Change-Id: Ib578b1747590bdf65327d4674e0849811ed999eb
Reviewed-on: https://gerrit.openafs.org/13701
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Yadavendra Yadav <yadayada@in.ibm.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>