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

Commit 71fbd556adde ("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: 71fbd556adde ("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>

drivers/base/memory.c

@@ -226,11 +226,9 @@ memory_block_action(unsigned long phys_index, unsigned long action, int online_t
 {
 	unsigned long start_pfn;
 	unsigned long nr_pages = PAGES_PER_SECTION * sections_per_block;
-	struct page *first_page;
 	int ret;
 
 	start_pfn = section_nr_to_pfn(phys_index);
-	first_page = pfn_to_page(start_pfn);
 
 	switch (action) {
 	case MEM_ONLINE: