btrfs: reduce extent tree lock contention when searching for inline backref

btrfs: reduce extent tree lock contention when searching for inline backref

When inserting extent backref, in order to check whether refs other than
inline refs are used, we always use path keep locks for tree search, which
will increase the lock contention of extent tree.

We do not need the parent node every time to determine whether normal
refs are used.  It is only needed when the extent item is the last item
in a leaf.

Therefore, we change it to first use keep_locks=0 for search.  If the
extent item happens to be the last item in the leaf, we then change to
keep_locks=1 for the second search to reduce lock contention.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>