btrfs: make wait_extent_bit() static

The function wait_extent_bit() is not used outside extent-io-tree.c so
make it static. Furthermore the function doesn't have the 'btrfs_' prefix.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>