bcachefs: Initialize padding space after alloc bkey

bcachefs: Initialize padding space after alloc bkey

Packed bkeys are padded up to 64 bit alignment, but the alloc bkey type
was not clearing the pad bytes after the last data byte. This left the
key possibly containing some random garbage at the end.

This problem was found using valgrind.

This patch also changes a path with the inode bkey to clear in the same
way.

Signed-off-by: Justin Husted <sigstop@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>