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

Fix this harmless build warning:

arch/arm/mm/alignment.c: In function 'do_alignment':
arch/arm/mm/alignment.c:749:21: warning: 'offset.un' may be used uninitialized in this function

This is caused by the compiler not being able to properly analyse the
code to prove that offset.un is assigned in every case.  The case it
struggles with is where we assign the handler from the Thumb parser -
do_alignment_t32_to_handler().  As this starts by zeroing this variable
via a pointer, move it into the calling function.  This fixes the
warning.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>