bcache: Add bch_keylist_init_single()

This will potentially save us an allocation when we've got inode/dirent bkeys
that don't fit in the keylist's inline keys.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --cc drivers/md/bcache/bset.h
Simple merge

diff --cc drivers/md/bcache/journal.c
Simple merge