efi: locking fix in efivar_entry_set_safe()

The intent is that if we aren't allowed to block because we're in an
NMI or an emergency then we only take the lock if it is uncontended.

Part of the problem is the test is reversed so we return -EBUSY if we
acquire the lock.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>