fscrypt: new helper function - fscrypt_require_key()

Add a helper function which checks if an inode is encrypted, and if so,
tries to set up its encryption key.  This is a pattern which is
duplicated in multiple places in each of ext4, f2fs, and ubifs --- for
example, when a regular file is asked to be opened or truncated.

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