Linux 5.16 compat: linux/elevator.h

Commit https://github.com/torvalds/linux/commit/2e9bc346 moved
the elevator.h header under the block/ directory as part of some
refactoring. This turns out not to be a problem since there's
no longer anything we need from the header. This has been the
case for some time, this change removes the elevator.h include
and replaces it with a major.h include.

Reviewed-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12725