bcachefs: Inode create optimization

bcachefs: Inode create optimization

On workloads that do a lot of multithreaded creates all at once, lock
contention on the inodes btree turns out to still be an issue.

This patch adds a small buffer of inode numbers that are known to be
free, so that we can avoid touching the btree on every create. Also,
this changes inode creates to update via the btree key cache for the
initial create.

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