mm: declare mpol_to_str() when CONFIG_TMPFS=n

When CONFIG_TMPFS=n mpol_to_str() is not declared in mempolicy.h.
However, in the NUMA case, the definition is always compiled.

Since it is not strictly true that tmpfs is the only client, and since the
symbol was always lurking around anyways, export mpol_to_str()
unconditionally.  Furthermore, this will allow us to move show_numa_map()
out of mempolicy.c and into the procfs subsystem.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>