BugLink: http://bugs.launchpad.net/bugs/1636517

Xenial kernel commit 193fb6a2c94fab8eb8ce70a5da4d21c7d4023bee
("block_dev: Support checking inode permissions in lookup_bdev()")
added a flags argument to block_dev which caused this breakage. Add
detection of 1 or 2 arg block_dev and add a zfs_block_dev shim to
abstract these differences away. Kudos to Fabian Grünbichler for
the original fix that this fix is based on.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>