mm/slub, kunit: Make slub_kunit unaffected by user specified flags

slub_kunit does not expect other debugging flags to be set when running
tests. When SLAB_RED_ZONE flag is set globally, test fails because the
flag affects number of errors reported.

To make slub_kunit unaffected by user specified debugging flags,
introduce SLAB_NO_USER_FLAGS to ignore them. With this flag, only flags
specified in the code are used and others are ignored.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/Yk0sY9yoJhFEXWOg@hyeyoo