bcachefs: Remove undefined behavior in bch2_dev_buckets_reserved()

bcachefs: Remove undefined behavior in bch2_dev_buckets_reserved()

In general it's a good idea to avoid using bare unreachable() because it
introduces undefined behavior in compiled code.  In this case it even
confuses GCC into emitting an empty unused
bch2_dev_buckets_reserved.part.0() function.

Use BUG() instead, which is nice and defined.  While in theory it should
never trigger, if something were to go awry and the BCH_WATERMARK_NR
case were to actually hit, the failure mode is much more robust.

Fixes the following warnings:

  vmlinux.o: warning: objtool: bch2_bucket_alloc_trans() falls through to next function bch2_reset_alloc_cursors()
  vmlinux.o: warning: objtool: bch2_dev_buckets_reserved.part.0() is missing an ELF size annotation

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>