Linux: increase max nvlist_src size

On Linux increase the maximum allowed size of the src nvlist which
can be passed to the /dev/zfs ioctl. Originally, this was set
to a maximum of KMALLOC_MAX_SIZE (4M) because it was kmalloc'd.
Since that time it's been converted to a vmalloc so that's no
longer a hard limit, and it's desirable for zfs send/recv to
allow larger nvlists so more snapshots can be sent at once.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6572
Closes #11638