shmem: avoid maybe-uninitialized warning

After enabling -Wmaybe-uninitialized warnings, we get a false-postive
warning for shmem:

  mm/shmem.c: In function `shmem_getpage_gfp':
  include/linux/spinlock.h:332:21: error: `info' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This can be easily avoided, since the correct 'info' pointer is known at
the time we first enter the function, so we can simply move the
initialization up.  Moving it before the first label avoids the warning
and lets us remove two later initializations.

Note that the function is so hard to read that it not only confuses the
compiler, but also most readers and without this patch it could\ easily
break if one of the 'goto's changed.

Link: https://www.spinics.net/lists/kernel/msg2368133.html
Link: http://lkml.kernel.org/r/20161024205725.786455-1-arnd@arndb.deSigned-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -1539,7 +1539,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	struct mm_struct *fault_mm, int *fault_type)
 {
 	struct address_space *mapping = inode->i_mapping;
-	struct shmem_inode_info *info;
+	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct shmem_sb_info *sbinfo;
 	struct mm_struct *charge_mm;
 	struct mem_cgroup *memcg;
@@ -1589,7 +1589,6 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	 * Fast cache lookup did not find it:
 	 * bring it back from swap or allocate.
 	 */
-	info = SHMEM_I(inode);
 	sbinfo = SHMEM_SB(inode->i_sb);
 	charge_mm = fault_mm ? : current->mm;
 
@@ -1837,7 +1836,6 @@ alloc_nohuge:		page = shmem_alloc_and_acct_page(gfp, info, sbinfo,
 		put_page(page);
 	}
 	if (error == -ENOSPC && !once++) {
-		info = SHMEM_I(inode);
 		spin_lock_irq(&info->lock);
 		shmem_recalc_inode(inode);
 		spin_unlock_irq(&info->lock);