LU-9859 libcfs: move tgt_descs to standard Linux bitmaps.

LU-9859 libcfs: move tgt_descs to standard Linux bitmaps.

Originally the Linux kernel was lacking a uniform bitmap API so
Lustre created its own. Todays modern kernels support a standard
bitmap API so migrate tgt_descs to the standard API.

Change-Id: If43b520c29d16355189b1eb7f9fdab7309446545
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/38981
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>