Remove zfs_sync() panicking kernel check

This check provides no real additional protection and unnecessarily
introduces a dependency on the "oops_in_progress" kernel symbol.
Remove the check, it there are special circumstances on other
platforms which make this a requirement it can be reintroduced
for all relevant call paths in a more portable comprehensive manor.

Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8297