Btrfs: exclude super blocks when we read in block groups

This has been resulting in a BUT_ON(ret) after btrfs_reserve_extent in
btrfs_cow_file_range.  The reason is we don't actually calculate the bytes_super
for a block group until we go to cache it, which means that the space_info can
hand out reservations for space that it doesn't actually have, and we can run
out of data space.  This is also a problem if you are using space caching since
we don't ever calculate bytes_super for the block groups.  So instead everytime
we read a block group call exclude_super_stripes, which calculates the
bytes_super for the block group so it can be left out of the space_info.  Then
whenever caching completes we just call free_excluded_extents so that the super
excluded extents are freed up.  Also if we are unmounting and we hit any block
groups that haven't been cached we still need to call free_excluded_extents to
make sure things are cleaned up properly.  Thanks,

Reported-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>