LU-2650 procfs: return -ENOMEM from lprocfs_register()

LU-2650 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.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I8d452781afe4b7c174c3b5fac9f4f0e573a3d85c
Reviewed-on: http://review.whamcloud.com/5161
Tested-by: Hudson
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>