xmalloc: stop using a magic '1' in alignment padding

xmalloc: stop using a magic '1' in alignment padding

Alignment padding inserts a pseudo block header in front of the allocation,
sets its size field to the pad size and then ORs in 1, which is equivalent
to marking it as a free block, so that xfree() can distinguish it from a
real block header.

This patch simply replaces the magic '1' with the defined 'FREE_BLOCK' to
make it more obvious what's going on. Also, whilst in the neighbourhood,
it removes a stray space after a cast.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>