btrfs-progs: check: fix the return value bug of cmd_check()

btrfs-progs: check: fix the return value bug of cmd_check()

The function cmd_check() is called by the main function of btrfs.c, its
return value will be returned by exit(). Resulting in the loss of
significant bits in some cases, for example this value is greater than
0377. If use a bool value "err" to store all of the return value, this
will solve the problem.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>