staging/lustre/lvfs: fix building without CONFIG_PROC_FS

Patch "staging/lustre/lvfs: remove the lvfs layer" moved some code around
that was previously only built when CONFIG_PROC_FS was set, which now
causes an invalid access to the obd_memory variable:

ERROR: "obd_memory" [drivers/staging/lustre/lustre/obdclass/obdclass.ko] undefined!

This fixes it by using adding a compile-time check around the access.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: fdedd945 ("staging/lustre/lvfs: remove the lvfs layer")
Cc: James Simmons <uja.ornl@gmail.com>
Cc: Mikhail Pershin <mike.pershin@intel.com>
Cc: John L. Hammond <john.hammond@intel.com>
Cc: Jian Yu <jian.yu@intel.com>
Cc: Bob Glossman <bob.glossman@intel.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>