Linux: drop GLOCK before calling dput

Linux: drop GLOCK before calling dput

Holding the GLOCK when calling dput can result in a deadlock when
the kernel calls back into afs_dentry_iput.  It should be safe
to drop the lock here.

Change-Id: I8966aea7336b39b4c3ec265a53d820fc6135eeaf
Reviewed-on: http://gerrit.openafs.org/8046
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>