Linux 6.14: BLK_MQ_F_SHOULD_MERGE was removed

According to the upstream change, all callers set it, and all block
devices either honoured it or ignored it, so removing it entirely allows
a bunch of handling for the "unset" case to be removed, and it becomes
effectively implied.

We follow suit, and keep setting it for older kernels.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>