bcachefs: Improve bch2_bkey_make_mut()

bcachefs: Improve bch2_bkey_make_mut()

bch2_bkey_make_mut() now takes the bkey_s_c by reference and points it
at the new, mutable key.

This helps in some fsck paths that may have multiple repair operations
on the same key.

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