Update the MOS directory on spa_upgrade_errlog()

spa_upgrade_errlog() does not update the MOS directory when the
head_errlog feature is enabled. In this case if spa_errlog_sync() is not
called, the MOS dir references the old errlog_last and errlog_sync
objects. Thus when doing a scrub a panic will occur:

Call Trace:
dump_stack+0x6d/0x8b
panic+0x101/0x2e3
spl_panic+0xcf/0x102 [spl]
delete_errlog+0x124/0x130 [zfs]
spa_errlog_sync+0x256/0x260 [zfs]
spa_sync_iterate_to_convergence+0xe5/0x250 [zfs]
spa_sync+0x2f7/0x670 [zfs]
txg_sync_thread+0x22d/0x2d0 [zfs]
thread_generic_wrapper+0x83/0xa0 [spl]
kthread+0x104/0x140
ret_from_fork+0x1f/0x40

Fix this by updating the related MOS directory objects in
spa_upgrade_errlog().

Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #15279
Closes #15277