LU-1484 lprocfs: refine LC_PROCFS_USERS check

LU-1484 lprocfs: refine LC_PROCFS_USERS check

In some RHEL patched 2.6.18 kernels, pde_users member is added in
another struct proc_dir_entry_aux instead of in struct proc_dir_entry
in later kernel version of 2.6.23.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Icee65893b2fbf4d0c3b3e957cb038be99aaf6eb8
Reviewed-on: http://review.whamcloud.com/3471
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>