btrfs-progs: check: return value of check_extent_refs

btrfs-progs: check: return value of check_extent_refs

In original check mode (without option --repair), check_extent_refs()
always returns 0.

Add a variable @err to record status while checking extents.  At the end
of check_extent_refs(), let it return -EIO if @err is non-zero.

The test fsck/006-bad-root-items will fail after this patch and fixed by
the following patches.

Example:
$ btrfs check bad-extent-inline-ref-type.raw
Checking filesystem on bad-extent-inline-ref-type.raw
UUID: 1942d6fe-617b-4499-9982-cc8ffae5447f
checking extents
corrupt extent record: key 29360128 169 16384
ref mismatch on [29360128 16384] extent item 0, found 1
Backref 29360128 parent 5 root 5 not found in extent tree
backpointer mismatch on [29360128 16384]
bad extent [29360128, 29376512), type mismatch with chunk
checking free space cache
checking fs roots
checking csums
checking root refs
found 114688 bytes used, no error found
total csum bytes: 0
total tree bytes: 114688
total fs tree bytes: 32768
total extent tree bytes: 16384
btree space waste bytes: 109471
file data blocks allocated: 0
 referenced 0

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
[ add note about the failing test, rename variable to err ]
Signed-off-by: David Sterba <dsterba@suse.com>