Linux: Avoid deadlock in readdir - release GLOCK for filldir

Linux: Avoid deadlock in readdir - release GLOCK for filldir

The GLOCK is held while calling the filldir function in afs_linux_readdir().
If this function causes a page fault, and in particular if this fault
involves AFS, we're in trouble as we'll eventually deadlock in the
readpage code.
A simple test case for this is to call the getdents syscall on an
AFS directory with a buffer that is part of an mmaped AFS file.

This is already the case in the master branch; the change was part of
the merge of the NFS translator code.

FIXES 125555

Change-Id: I829838e45f94921d22335154587216f7842e3955
Reviewed-on: http://gerrit.openafs.org/760
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>