fscrypt: fix build with pre-4.6 gcc versions

fscrypt: fix build with pre-4.6 gcc versions

gcc versions prior to 4.6 require an extra level of braces when using a
designated initializer for a member in an anonymous struct or union.
This caused a compile error with the 'struct qstr' initialization in
__fscrypt_encrypt_symlink().

Fix it by using QSTR_INIT().

Reported-by: Andrew Morton <[email protected]>
Fixes: 76e81d6d5048 ("fscrypt: new helper functions for ->symlink()")
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>