As part of an effort to clean up fscrypt-related error codes, make
FS_IOC_SET_ENCRYPTION_POLICY fail with EEXIST when the file already uses
a different encryption policy.  This is more descriptive than EINVAL,
which was ambiguous with some of the other error cases.

I am not aware of any users who might be relying on the previous error
code of EINVAL, which was never documented anywhere.

This failure case will be exercised by an xfstest.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>