Advance the maximum number of arguments from 9 to 15 to account for
all potential feature flags that may be supplied.

Linux 4.19 added "meta_device"
(356d9d52) and "recalculate"
(a3fcf725) flags.

Commit 468dfca3 added
"sectors_per_bit" and "bitmap_flush_interval".

Commit 84597a44 added
"allow_discards".

And the commit d537858a added
"fix_padding".
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>