LU-9245 utils: fix string copy handling within lgss_sk

LU-9245 utils: fix string copy handling within lgss_sk

Strings are not copied correctly when the fsname or nodemap field of
a shared key is replaced. Instead of being zero-terminated, the new
string may overlap the original if it is smaller.

Fixed by using the buffer length instead of the string length in
strncpy().

Change-Id: I17bb5aa0ca7c25b2545c17c6f23e69045730a547
Test-Parameters: trivial
Signed-off-by: Chris Hanna <hannac@iu.edu>
Reviewed-on: https://review.whamcloud.com/26165
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Kit Westneat <kit.westneat@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>