fault_in_multipages_readable() throws set-but-unused error

When building XFS with -Werror, it now fails with:

include/linux/pagemap.h: In function 'fault_in_multipages_readable':
include/linux/pagemap.h:602:16: error: variable 'c' set but not used [-Werror=unused-but-set-variable]
volatile char c;
^

This is a regression caused by commit e23d4159b109 ("fix
fault_in_multipages_...() on architectures with no-op access_ok()").
Fix it by re-adding the "(void)c" trick taht was previously used to make
the compiler think the variable is used.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>