staging/lustre/procfs: return -ENOMEM from lprocfs_register()

In lprocfs_register(), if proc_mkdir() fails then return
ERR_PTR(-ENOMEM) rather than NULL and hold _lprocfs_mutex for the
whole function.  In lprocfs_remove_nolock() return early if the entry
is an error pointer. Improve error handling around lprocfs_register()
in a few spots.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2650
Lustre-change: http://review.whamcloud.com/5161


Signed-off-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>