LU-5153 fld: lsf_lock in update_from_controller

LU-5153 fld: lsf_lock in update_from_controller

The lsf_lock mutex must be held for calls to
fld_index_create, and is currently taken before calls to
fld_insert_entry.

This patch adds a comment and an assertion to
fld_insert_entry, and takes the mutex for the call in
fld_update_from_controller.

Also adds subsystem description from Di Wang to header file.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I3dcc9a53589d128343d9f386eaf7265f39ff5840
Reviewed-on: http://review.whamcloud.com/10631
Tested-by: Jenkins
Reviewed-by: wangdi <di.wang@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>