xen/arm: traps: Avoid BUG_ON() in do_trap_brk()

xen/arm: traps: Avoid BUG_ON() in do_trap_brk()

At the moment, do_trap_brk() is using a BUG_ON() to check the hardware
has been correctly configured during boot.

Any error when configuring the hardware could result to a guest 'brk'
trapping in the hypervisor and crash it.

This is pretty harsh to kill Xen when actually killing the guest would
be enough as misconfiguring this trap would not lead to exposing
sensitive data. Replace the BUG_ON() with crashing the guest.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>