bcachefs: Don't use sha256 for siphash str hash key

bcachefs: Don't use sha256 for siphash str hash key

With the refactoring that's coming to add fuse support, we want
bch2_hash_info_init() to be cheaper so we don't have to rely on anything
cached besides the inode in the btree.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>