bcachefs: Fix nonce inconsistency in bch2_write_prep_encoded_data()

bcachefs: Fix nonce inconsistency in bch2_write_prep_encoded_data()

If we're moving an extent that was partially overwritten,
bch2_write_rechecksum() will trim it to the currenty live range.

If we then also want to compress it, it'll be decrypted - but the nonce
has been advanced for the overwritten start of the extent that we
dropped, and we were using the nonce we calculated before rechecksum().

Reported-by: Gabriel de Perthuis <g2p.code@gmail.com>
Fixes: 127d90d2823e ("bcachefs: bch2_write_prep_encoded_data() now returns errcode")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>