xen/xmalloc: XMEM_POOL_POISON improvements

xen/xmalloc: XMEM_POOL_POISON improvements

When in use, the spew:

  (XEN) Assertion '!memchr_inv(b->ptr.buffer + MIN_BLOCK_SIZE, POISON_BYTE, (b->size & BLOCK_SIZE_MASK) - MIN_BLOCK_SIZE)' failed at common/xmalloc_tlsf.c:246

is unweidly and likely meaningless to non-Xen developers.  Therefore:

 * Switch to IS_ENABLED().  There's no need for full #ifdef-ary.
 * Pull memchr_inv() out into the if(), and provide an error message which
   clearly states that corruption has been found.
 * XMEM_POOL_POISON can be enabled in release builds.  Use printk()+BUG() so
   it doesn't silently stop working when assertions are compiled out.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>