drivers/base/memory.c: Remove unused 'first_page' variable

Commit 71fbd556 ("memory-hotplug: remove redundant call of page_to_pfn")
introduced an optimization that rendered 'struct page* first_page'
useless in memory_block_action(). Compiling with W=1 gives the
following warning, fix it.

drivers/base/memory.c: In function ‘memory_block_action’:
drivers/base/memory.c:229:15: warning: variable ‘first_page’ set but not used [-Wunused-but-set-variable]
  struct page *first_page;
               ^

This is a harmeless warning and is only being fixed to reduce the
noise with W=1 in the kernel. The call to pfn_to_page() has no side
effects and is safe to remove.

Fixes: 71fbd556

 ("memory-hotplug: remove redundant call of page_to_pfn")
Cc: Zhang Zhen <zhenzhang.zhang@huawei.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Kirtika Ruchandani <kirtika@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>