bch2_varint_decode can do reads up to 7 bytes past the end ptr, for the
sake of performance - these extra bytes are always masked off.

This won't be a problem in practice if we make sure to burn 8 bytes in
any buffer that has bkeys in it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>