Btrfs: add support for asserts

One of the complaints we get a lot is how many BUG_ON()'s we have.  So to help
with this I'm introducing a kconfig option to enable/disable a new ASSERT()
mechanism much like what XFS does.  This will allow us developers to still get
our nice panics but allow users/distros to compile them out.  With this we can
go through and convert any BUG_ON()'s that we have to catch actual programming
mistakes to the new ASSERT() and then fix everybody else to return errors.  This
will also allow developers to leave sanity checks in their new code to make sure
we don't trip over problems while testing stuff and vetting new features.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>