Btrfs-progs: fix free space cache checks

This fixes two bugs with the free space cache checker.  First is we apparently
always use root->sectorsize for our unit in the kernel so we have to do that in
progs otherwise bitmaps turn out to not look right if we have leafsize !=
sectorsize.  The second is a small issue if we had skinny metadata extents set,
we wouldn't advance last properly because we unconditionally use key.offset
instead of root->leafsize.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Btrfs-progs: fix the allocator

The allocator looks for these hints when moving on to another block group which
will make it reset which block group it looks at, when we've already searched
that block group and didn't find any space to allocate, we need to fix this by
just letting the allocator make the determination if the block group is good
enough.  This also fixes a problem where if we couldn't find space in the block
group we were given we'd just error out instead of moving on to the next block
group.  Previously I couldn't fix some file systems that were relatively full,
but with this patch I can now run fsck on them with no allocation errors.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Btrfs-progs: fix fsck dealing with finding backrefs first

There is a problem where if we find a backref extent record first that doesn't
match a extent item we will delete some of the duplicates but not others.  In
order to deal with this we need to make sure we only pay attention to duplicates
that actually have duplicate extent items.  If a extent_rec has a duplicate but
the record itself doesn't have an associated extent item we promote the
duplicate to the extent record and just discard the original extent_rec since it
was just added by the backref.  We copy the backref onto the promoted extent
record and then continue processing.  This allowed me to fix a file system that
previously was not able to be fixed by fsck.  Thanks,

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