libmount: add fsconfig() btrfs workaround

libmount: add fsconfig() btrfs workaround

The new kernel mount API is not properly implemented in all FS
drivers. See strace output:

fsconfig(3, FSCONFIG_SET_STRING, "source", "/dev/sda", 0) = 0
fsconfig(3, FSCONFIG_SET_STRING, "context", "system_u:object_r:root_t:s0", 0) = 0
fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = -1 EINVAL (Invalid argument)

Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Karel Zak <kzak@redhat.com>