btrfs: introduce btrfs_try_lock_balance

This function contains the factored out locking sequence of
btrfs_ioctl_balance. Having this piece of code separate helps to
simplify btrfs_ioctl_balance which has too complicated.  This will be
used in the next patch to streamline the logic in btrfs_ioctl_balance.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>