include/linux/mm.h: remove ifdef condition

The ifdef conditions in include/linux/mm.h presents three cases:

 - !defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP) && !defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID)

   There is no actual definition of function but include/linux/mm.h has a
   static inline stub defined.

 - defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP) && !defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID)

   linux/mm.h does not define a prototype, but mm/page_alloc.c defines
   the function.

   Hence, compiler reports the following warning:

     mm/page_alloc.c:4300:15: warning: no previous prototype for `__early_pfn_to_nid' [-Wmissing-prototypes]

 - defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID)

   The architecture defines the function, and linux/mm.h has a
   prototype.

Thus, join the conditions of Case 2 and 3 ie eliminate the ifdef
condition of CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID to eliminate the missing
prototype warning from file mm/page_alloc.c.

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>