ARM: Fix another build warning in arch/arm/mm/alignment.c

One such warning was recently fixed in a761cebf "ARM: Fix build warning
in arch/arm/mm/alignment.c" but only for the thumb2 case, this fixes
the other half.

arch/arm/mm/alignment.c: In function 'do_alignment':
arch/arm/mm/alignment.c:327:15: error: 'offset.un' may be used uninitialized in this function
arch/arm/mm/alignment.c:748:21: note: 'offset.un' was declared here

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>