The new convention is that functions that handle transaction restarts
within an existing transaction context should return
-BCH_ERR_transaction_restart_nested when they did so, since they
invalidated the outer transaction context.

This also means bch2_btree_delete_range_trans() is changed to only call
bch2_trans_begin() after a transaction restart, not on every loop
iteration.

This is to fix a bug in fsck, in check_inode() when we truncate an inode
with BCH_INODE_I_SIZE_DIRTY set.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>