Btrfs: free space cache cleanups

This patch cleans up the free space cache code a bit.  It better documents the
idiosyncrasies of tree_search_offset and makes the code make a bit more sense.
I took out the info allocation at the start of __btrfs_add_free_space and put it
where it makes more sense.  This was left over cruft from when alloc_mutex
existed.  Also all of the re-searches we do to make sure we inserted properly.

Signed-off-by: Josef Bacik <jbacik@redhat.com>

diff --cc fs/btrfs/extent-tree.c
Simple merge

diff --cc fs/btrfs/free-space-cache.c
Simple merge