ext4 crypto: fix sparse warnings in fs/ext4/ioctl.c

[ Added another sparse fix for EXT4_IOC_GET_ENCRYPTION_POLICY while
  we're at it. --tytso ]

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>
Change-Id: I56b8943d83eb2a82e0b5c1544df3e70efe8e9525