Do not call afs_FlushVCBs with afs_xvcache held

Do not call afs_FlushVCBs with afs_xvcache held

In afs_AllocCBR, use dynamically created afs_cbr structures
when running out of preformatted ones, rather than calling
afs_FlushVCBs under, potentially, the afs_xvcache lock
(which would be held across the RPC, difficult to drop and
re-acquire under the current hierarchy).

Modest modernization of the number preformatted afs_cbr structures,
rule-of-thumb, not pretending any research.

Change-Id: I9427427d5dab7d4639822c370bdded0418f67d9e
Reviewed-on: http://gerrit.openafs.org/2243
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>