mm: compaction: remove redundant VM_BUG_ON() in compact_zone()

mm: compaction: remove redundant VM_BUG_ON() in compact_zone()

Patch series "Some small improvements for compaction".

When I did some compaction testing, I found some small room for
improvement as well as some code cleanups.

This patch (of 5):

The compaction_suitable() will never return values other than
COMPACT_SUCCESS, COMPACT_SKIPPED and COMPACT_CONTINUE, so after validation
of COMPACT_SUCCESS and COMPACT_SKIPPED, we will never hit other unexpected
case.  Thus remove the redundant VM_BUG_ON() validation for the return
values of compaction_suitable().

Link: https://lkml.kernel.org/r/cover.1673342761.git.baolin.wang@linux.alibaba.com
Link: https://lkml.kernel.org/r/740a2396d9b98154dba76e326cba5e798b640ead.1673342761.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>