mm/sparsemem: fix 'mem_section' will never be NULL gcc 12 warning

mm/sparsemem: fix 'mem_section' will never be NULL gcc 12 warning

The gcc 12 compiler reports a "'mem_section' will never be NULL" warning
on the following code:

    static inline struct mem_section *__nr_to_section(unsigned long nr)
    {
    #ifdef CONFIG_SPARSEMEM_EXTREME
        if (!mem_section)
                return NULL;
    #endif
        if (!mem_section[SECTION_NR_TO_ROOT(nr)])
                return NULL;
       :

It happens with both CONFIG_SPARSEMEM_EXTREME on and off.  The mem_section
definition is

    #ifdef CONFIG_SPARSEMEM_EXTREME
    extern struct mem_section **mem_section;
    #else
    extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
    #endif

In the CONFIG_SPARSEMEM_EXTREME case, mem_section obviously cannot be
NULL, but *mem_section can be if memory hasn't been allocated for the
dynamic mem_section[] array yet.  In the !CONFIG_SPARSEMEM_EXTREME case,
mem_section is a static 2-dimensional array and so the check
"!mem_section[SECTION_NR_TO_ROOT(nr)]" doesn't make sense.

Fix this warning by checking for "!*mem_section" instead of "!mem_section"
and moving the "!mem_section[SECTION_NR_TO_ROOT(nr)]" check up inside the
CONFIG_SPARSEMEM_EXTREME block.

Link: https://lkml.kernel.org/r/20220201192924.672675-1-longman@redhat.com
Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
Fixes: 3e347261a80b ("sparsemem extreme implementation")
Signed-off-by: Waiman Long <longman@redhat.com>
Reported-by: Justin Forbes <jforbes@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Justin Forbes <jforbes@redhat.com>
Cc: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>