LU-13783 o2iblnd: make FMR-pool support optional.

LU-13783 o2iblnd: make FMR-pool support optional.

Linux 5.8 removes the FMR-pool API.  This patch makes
all use for this API optional, selected only if the
support exists in the kernel.

Lustre-commit: 14b20ca66b2b6c5a735d39b753ec77fa6a574a6b
Lustre-change: https://review.whamcloud.com/40287

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I4c40f3a766f5b46ae4f26d7d3ecf8434a6e5a0cb
Reviewed-on: https://review.whamcloud.com/41152
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>