This introduces two new macros for iterating through the btree, with
transaction restart handling
 - for_each_btree_key2()
 - for_each_btree_key_commit()

Every iteration is now in an implicit transaction, and - as with
lockrestart_do() and commit_do() - returning -EINTR will cause the
transaction to be restarted, at the same key.

This patch converts a bunch of code that was open coding this to these
new macros, saving a substantial amount of code.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>