f2fs/015: clear MKFS_OPTIONS and MOUNT_OPTIONS

f2fs/015: clear MKFS_OPTIONS and MOUNT_OPTIONS

Currently, residual options can interfere with checking the mount
behavior of the listed MKFS_OPTIONS and MOUNT_OPTIONS. For example,
"Option#120: test_dummy_encryption" should fail with the listed
options but succeeds if “MKFS_OPTIONS=encrypt” is preset. By
explicitly clearing MKFS_OPTIONS and MOUNT_OPTIONS, the test’s
reliability can be improved.

f2fs/015.out is also updated to expect failure for cases that require
additional mkfs attributes to mount.

Signed-off-by: Joanne Chang <joannechien@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>