bcachefs: properly initialize used values

bcachefs: properly initialize used values

 - Ensure the second key value in bch_hash_info is initialized to zero
   if the info type is of type BCH_STR_HASH_SIPHASH.

 - Initialize the possibly returned value in bch2_inode_create. Assuming
   bch2_btree_iter_peek returns bkey_s_c_null, the uninitialized value
   of ret could be returned to the user as an error pointer.

 - Fix compiler warning in initialization of bkey_s_c_stripe

fs/bcachefs/buckets.c:1646:35: warning: suggest braces around initialization
of subobject [-Wmissing-braces]
        struct bkey_s_c_stripe new_s = { NULL };
                                         ^~~~

Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>