bcachefs: Track maximum transaction memory

bcachefs: Track maximum transaction memory

This patch
 - tracks maximum bch2_trans_kmalloc() memory used in btree_transaction_stats
 - makes it available in debugfs
 - switches bch2_trans_init() to using that for the amount of memory to
   preallocate, instead of the parameter passed in

This drastically reduces transaction restarts, and means we no longer
need to track this in the source code.

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