staging/lustre/libcfs: remove cfs_strncasecmp

The only difference against generic strncasecmp() is that
cfs_strncasecmp() check NULL pointers and zero length parameter,
which all callers have already taken care of.

Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>