keyring: cast FS_IOC_REMOVE_ENCRYPTION_KEY to uintptr (#221)
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.]

google/fscrypt,google/fscrypt,google/fscrypt