mm/shmem.c: distribute switch variables for initialization

Variables declared in a switch statement before any case statements cannot
be automatically initialized with compiler instrumentation (as they are
not part of any execution flow).  With GCC's proposed automatic stack
variable initialization feature, this triggers a warning (and they don't
get initialized).  Clang's automatic stack variable initialization (via
CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also doesn't
initialize such variables[1].  Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase, so
even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where they're
used or lift them up into the main function body.

mm/shmem.c: In function `shmem_getpage_gfp':
mm/shmem.c:1816:10: warning: statement will never be executed [-Wswitch-unreachable]
 1816 |   loff_t i_size;
      |          ^~~~~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Alexander Potapenko <glider@google.com>
Link: http://lkml.kernel.org/r/20200220062312.69165-1-keescook@chromium.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -1812,17 +1812,20 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	if (shmem_huge == SHMEM_HUGE_FORCE)
 		goto alloc_huge;
 	switch (sbinfo->huge) {
-		loff_t i_size;
-		pgoff_t off;
 	case SHMEM_HUGE_NEVER:
 		goto alloc_nohuge;
-	case SHMEM_HUGE_WITHIN_SIZE:
+	case SHMEM_HUGE_WITHIN_SIZE: {
+		loff_t i_size;
+		pgoff_t off;
+
 		off = round_up(index, HPAGE_PMD_NR);
 		i_size = round_up(i_size_read(inode), PAGE_SIZE);
 		if (i_size >= HPAGE_PMD_SIZE &&
 		    i_size >> PAGE_SHIFT >= off)
 			goto alloc_huge;
-		/* fallthrough */
+
+		fallthrough;
+	}
 	case SHMEM_HUGE_ADVISE:
 		if (sgp_huge == SGP_HUGE)
 			goto alloc_huge;