LINUX: Avoid duplicate mntget in afs_linux_raw_open

In the unlikely event that our afs_dentry_open call fails with
cache_creds, we call afs_dentry_open again with the current creds as a
fallback. However, we call mntget on afs_cacheMnt for each call. So if
we actually hit the second call, we'll have added 2 refs to
afs_cacheMnt, but we only actually opened one file, causing a slight
overcount on afs_cacheMnt refs.

To avoid this, just call mntget once, before any of the
dentry_open-related calls.

Change-Id: I7ec3e8c193dd7782ab629fb5d7615d83f8385b6c
Reviewed-on: http://gerrit.openafs.org/9791
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>