Revert "LINUX: Avoid duplicate mntget in afs_linux_raw_open"

Revert "LINUX: Avoid duplicate mntget in afs_linux_raw_open"

This reverts commit 5ccbbda19f11e7027300409c46715155f439424a.

dentry_open, at least on older kernels, decs the refcount on its
arguments in the case of an error. So calling mntget for each
dentry_open invocation actually is the correct thing to do.

This code may need to be further fixed in order to work for newer
kernels, but for now, at least put it back the way it was so we don't
undercount ref counts on older kernels.

Change-Id: Ie5e064b065bc41321c1cea8e0272cc4ea697e3b5
Reviewed-on: http://gerrit.openafs.org/9800
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>