Linux: use DEFINE_MUTEX for afs_linux_alloc_sem on newer kernels

Linux: use DEFINE_MUTEX for afs_linux_alloc_sem on newer kernels

DECLARE_MUTEX is gone as of 2.6.37; afs_global_lock switches from
DECLARE_MUTEX to DEFINE_MUTEX at 2.6.16, so do the same here.

Change-Id: If2ea5e1d3b5406c631cade1128425b5be79ffd4a
Signed-off-by: Alejandro R. Sedeño <asedeno@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3273
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>