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This fixes a null ptr deref when creating new snapshots:
bch2_create_trans() will lookup the subvolume and find the _new_
snapshot in the BCH_CREATE_SUBVOL path that's being created in that
transaction.

We have to call bch2_mark_snapshot() earlier so that it's properly
initialized, instead of leaving it for transaction commit.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>