btrfsck: decode error properly

check_mounted() returns kernel-style negative errors.
Patch drops sign for strerror().

Before the patch:
check_mounted(): Could not open /dev/sdb2
Could not check mount status: Unknown error 18446744073709551603

After the patch:
check_mounted(): Could not open /dev/sdb2
Could not check mount status: Permission denied

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
3 files changed