lib/show_mem.c: drop pgdat_resize_lock in show_mem()

lib/show_mem.c: drop pgdat_resize_lock in show_mem()

Function show_mem() is used to print system memory status when user
requires or fail to allocate memory.  Generally, this is a best effort
information so any races with memory hotplug (or very theoretically an
early initialization) should be tolerable and the worst that could happen
is to print an imprecise node state.

Drop the resize lock because this is the only place which might hold the
lock from the interrupt context and so all other callers might use a
simple spinlock.  Even though this doesn't solve any real issue it makes
the code easier to follow and tiny more effective.

Link: http://lkml.kernel.org/r/20181129235532.9328-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>