efivarfs: Drop link count of the right inode

efivarfs_unlink() should drop the file's link count, not the directory's.

Signed-off-by: Lingzhu Xiang <lxiang@redhat.com>
Cc: Jeremy Kerr <jeremy.kerr@canonical.com>
Tested-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>