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

man/man5/zfs-module-parameters.5

@@ -1186,7 +1186,7 @@ amount of memory. When the limit is exceeded, the ioctl fails with EINVAL and a
 description of the error is sent to the zfs-dbgmsg log. This parameter should
 not need to be touched under normal circumstances. On FreeBSD, the default is
 based on the system limit on user wired memory. On Linux, the default is
-\fBKMALLOC_MAX_SIZE\fR .
+\fB128MB\fR.
 .sp
 Default value: \fB0\fR (kernel decides)
 .RE

module/os/linux/zfs/zfs_ioctl_os.c

@@ -209,7 +209,7 @@ zfs_max_nvlist_src_size_os(void)
 	if (zfs_max_nvlist_src_size != 0)
 		return (zfs_max_nvlist_src_size);
 
-	return (KMALLOC_MAX_SIZE);
+	return (MIN(ptob(zfs_totalram_pages) / 4, 128 * 1024 * 1024));
 }
 
 void