Linux 5.10 compat: also zvol_revalidate_disk()

Commit 59b68723 added a configure check for 5.10, which removed
revalidate_disk(), and conditionally replaced it's usage with a call to
the new revalidate_disk_size() function. However, the old function also
invoked the device's registered callback, in our case
zvol_revalidate_disk(). This commit adds a call to zvol_revalidate_disk()
in zvol_update_volsize() to make sure the code path stays the same.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Michael D Labriola <michael.d.labriola@gmail.com>
Closes #11358