mm, fault-injection: declare should_fail_alloc_page()

mm, fault-injection: declare should_fail_alloc_page()

The mm/ directory can almost fully be built with W=1, which would help
in local development.  One remaining issue is missing prototype for
should_fail_alloc_page().  Thus add it next to the should_failslab()
prototype.

Note the previous attempt by commit f7173090033c ("mm/page_alloc: make
should_fail_alloc_page() static") had to be reverted by commit
54aa386661fe as it caused an unresolved symbol error with
CONFIG_DEBUG_INFO_BTF=y

Link: https://lkml.kernel.org/r/20220314165724.16071-1-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>