ARM: fix warning on uninitialized offset.un

This patch fixes the following warning:

arch/arm/mm/alignment.c: In function 'do_alignment':
arch/arm/mm/alignment.c:327:15: warning: 'offset.un' may be used uninitialized in this function [-Wmaybe-uninitialized]
arch/arm/mm/alignment.c:749:21: note: 'offset.un' was declared here

The fix is to promote the initialization of offset into do_alignment().

Signed-off-by: Cyril Chemparathy <cyril@ti.com>