Don't hold on to the afs_xvcache lock while creating a symlink

Don't hold on to the afs_xvcache lock while creating a symlink

Holding the afs_xvcache lock over the RPC while creating a symlink is a
severe blow to performance as look-ups from other tasks likely
require the lock. Drop the lock and rely solely on the fileserver
to detect a clash with a same entry created simultaneously.

Change-Id: Ia6de353bd9dc003addf5f3be7a8f3e769247a513
Reviewed-on: http://gerrit.openafs.org/2233
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>