LU-14677 sec: no encryption key migrate/extend/resync/split

LU-14677 sec: no encryption key migrate/extend/resync/split

Allow some layout operations on encrypted files, even when the
encryption key is not available:
- lfs migrate
- lfs mirror extend
- lfs mirror resync
- lfs mirror verify
- lfs mirror split
We allow these access patterns to applications that know what they are
doing, by using the specific flags O_FILE_ENC or O_FILE_ENC_NONDIRECT.
O_FILE_ENC_NONDIRECT has the same semantic as O_FILE_ENC, but it
allows access without O_DIRECT. This is reserved for cases where
cached data is wiped once IOs are finished, for instance via fadvise.

Also update sanity-sec test_40 and test_52 to exercise these access
patterns.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ieaeee0e5bf7643f18d775fe6daa5e31c2f349f8c