bcachefs: Better approach to write vs. read lock deadlocks

bcachefs: Better approach to write vs. read lock deadlocks

Instead of unconditionally upgrading read locks to intent locks in
do_bch2_trans_commit(), this patch changes the path that takes write
locks to first trylock, and then if trylock fails check if we have a
conflicting read lock, and restart the transaction if necessary.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>