xen/sched: Improvements to the {alloc,free}_domdata() interfaces

xen/sched: Improvements to the {alloc,free}_domdata() interfaces

The main purpose of this change is for the subsequent cleanup it enables, but
it stands on its own merits.

In principle, these hooks are optional, but the SCHED_OP() default aliases a
memory allocation failure, which causes arinc653 to play the dangerous game of
passing its priv pointer back, and remembering not to actually free it.

Redefine alloc_domdata to use ERR_PTR() for errors, NULL for nothing, and
non-NULL for a real allocation, which allows the hook to become properly
optional.  Redefine free_domdata to be idempotent.

For arinc653, this means the dummy hooks can be dropped entirely.  For the
other schedulers, this means returning ERR_PTR(-ENOMEM) instead of NULL for
memory allocation failures, and modifying the free hooks to cope with a NULL
pointer.  While making the alterations, drop some spurious casts to void *.

Introduce and use proper wrappers for sched_{alloc,free}_domdata().  These are
strictly better than SCHED_OP(), as the source code is visible to
grep/cscope/tags, the generated code is better, and there can be proper
per-hook defaults and checks.

Callers of the alloc hooks are switched to using IS_ERR(), rather than
checking for NULL.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Meng Xu <mengxu@cis.upenn.edu>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>