LU-16667 build: kernel_cap_t contains u64

LU-16667 build: kernel_cap_t contains u64

linux kernel v6.2-13111-gf122a08b197d
  capability: just use a 'u64' instead of a 'u32[2]' array

Add configure test for kernel_cap_t as u64 and provide
and accessor for the least significant 32 bits.

As of linux commit v3.6-10973-g607ca46e97a1 lustre implicitly
started to ignore some capabilities, see:
   include/uapi/linux/capability.h

The last capability flag was added by:
   linux commit v5.8-rc5-1-g124ea650d307

The capabilities the Lustre currently ignores are:
 - CAP_MAC_OVERRIDE
 - CAP_MAC_ADMIN
 - CAP_SYSLOG
 - CAP_WAKE_ALARM
 - CAP_BLOCK_SUSPEND
 - CAP_AUDIT_READ
 - CAP_PERFMON
 - CAP_BPF
 - CAP_CHECKPOINT_RESTORE

None of which appear to be important to Lustre operations
and should be fine to continue ignore.

Test-Parameters: trivial
HPE-bug-id: LUS-11557
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I48ad7b1a34fff378c260dc73ea91b22aaa0d7469
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50421
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>