generic: test encryption key revocation during concurrent I/O

generic: test encryption key revocation during concurrent I/O

Add a test which revokes a keyring key while other processes are
performing I/O on an encrypted file that was "unlocked" using that key.
The crashes unpatched kernels with filesystem encryption enabled.

This bug was present in kernels v4.2 and later.  It has been fixed in
v4.11-rc4, v4.10.7, v4.9.20, and v4.4.59.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Michael Halcrow <mhalcrow@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>