kvm: arm64: use BUG() instead of BUG_ON(1)

kvm: arm64: use BUG() instead of BUG_ON(1)

The BUG_ON() macro adds a little bit of complexity over BUG(), and in
some cases this ends up confusing the compiler's control flow analysis
in a way that results in a warning. This one now shows up with clang-21:

arch/arm64/kvm/vgic/vgic-mmio.c:1094:3: error: variable 'len' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
 1094 |                 BUG_ON(1);

Change both instances of BUG_ON(1) to a plain BUG() in the arm64 kvm
code, to avoid the false-positive warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250807072132.4170088-1-arnd@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>