keyring: cast FS_IOC_REMOVE_ENCRYPTION_KEY to uintptr (#221)

Since v0.2.6, fscrypt only builds for 64-bit systems.  E.g. trying to
build on Raspbian fails with the following error:

  $ go get github.com/google/fscrypt/cmd/fscrypt
  # github.com/google/fscrypt/keyring
  go/src/github.com/google/fscrypt/keyring/fs_keyring.go:231:6: constant 3225445912 overflows int
  go/src/github.com/google/fscrypt/keyring/fs_keyring.go:235:7: constant 3225445913 overflows int

Fix it by making the 'ioc' variable have type uintptr.

[EB - removed the later cast to uintptr that became unnecessary, and
 added explanation to commit message.]

keyring/fs_keyring.go

@@ -228,7 +228,7 @@ func fsRemoveEncryptionKey(descriptor string, mount *filesystem.Mount,
 		return err
 	}
 
-	ioc := unix.FS_IOC_REMOVE_ENCRYPTION_KEY
+	ioc := uintptr(unix.FS_IOC_REMOVE_ENCRYPTION_KEY)
 	iocName := "FS_IOC_REMOVE_ENCRYPTION_KEY"
 	var savedPrivs *savedPrivs
 	if user == nil {
@@ -240,7 +240,7 @@ func fsRemoveEncryptionKey(descriptor string, mount *filesystem.Mount,
 			return err
 		}
 	}
-	_, _, errno := unix.Syscall(unix.SYS_IOCTL, dir.Fd(), uintptr(ioc), uintptr(unsafe.Pointer(&arg)))
+	_, _, errno := unix.Syscall(unix.SYS_IOCTL, dir.Fd(), ioc, uintptr(unsafe.Pointer(&arg)))
 	restorePrivs(savedPrivs)
 
 	log.Printf("%s(%q, %s) = %v, removal_status_flags=0x%x",