LINUX: Avoid duplicate mntget in afs_dentry_open

Our caller already got a ref to 'mnt'; getting another one here will
overcount refs on 'mnt'.

This can make it impossible to unmount the filesystem the cache
resides on (even after the client is stopped), since it's ref count
will be very high.

FIXES 131613

Reviewed-on: http://gerrit.openafs.org/9790
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit e019429d4548348c6ab8674305d985feee040476)

Change-Id: If88e108bb3e67602cf9e6bbe7646f9862228b684
Reviewed-on: http://gerrit.openafs.org/9792
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>