Fix /etc/hostid on root pool deadlock

Accidentally introduced by dc04a8c which now takes the SCL_VDEV lock
as a reader in zfs_blkptr_verify().  A deadlock can occur if the
/etc/hostid file resides on a dataset in the same pool.  This is
because reading the /etc/hostid file may occur while the caller is
holding the SCL_VDEV lock as a writer.  For example, to perform a
`zpool attach` as shown in the abbreviated stack below.

To resolve the issue we cache the system's hostid when initializing
the spa_t, or when modifying the multihost property.  The cached
value is then relied upon for subsequent accesses.

Call Trace:
    spa_config_enter+0x1e8/0x350 [zfs]
    zfs_blkptr_verify+0x33c/0x4f0 [zfs] <--- trying read lock
    zio_read+0x6c/0x140 [zfs]
    ...
    vfs_read+0xfc/0x1e0
    kernel_read+0x50/0x90
    ...
    spa_get_hostid+0x1c/0x38 [zfs]
    spa_config_generate+0x1a0/0x610 [zfs]
    vdev_label_init+0xa0/0xc80 [zfs]
    vdev_create+0x98/0xe0 [zfs]
    spa_vdev_attach+0x14c/0xb40 [zfs] <--- grabbed write lock

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9256 
Closes #9285

module/zfs/spa_misc.c

@@ -668,6 +668,7 @@ spa_add(const char *name, nvlist_t *config, const char *altroot)
 	spa->spa_proc = &p0;
 	spa->spa_proc_state = SPA_PROC_NONE;
 	spa->spa_trust_config = B_TRUE;
+	spa->spa_hostid = zone_get_hostid(NULL);
 
 	spa->spa_deadman_synctime = MSEC2NSEC(zfs_deadman_synctime_ms);
 	spa->spa_deadman_ziotime = MSEC2NSEC(zfs_deadman_ziotime_ms);
@@ -2560,22 +2561,10 @@ spa_multihost(spa_t *spa)
 	return (spa->spa_multihost ? B_TRUE : B_FALSE);
 }
 
-unsigned long
-spa_get_hostid(void)
+uint32_t
+spa_get_hostid(spa_t *spa)
 {
-	unsigned long myhostid;
-
-#ifdef	_KERNEL
-	myhostid = zone_get_hostid(NULL);
-#else	/* _KERNEL */
-	/*
-	 * We're emulating the system's hostid in userland, so
-	 * we can't use zone_get_hostid().
-	 */
-	(void) ddi_strtoul(hw_serial, NULL, 10, &myhostid);
-#endif	/* _KERNEL */
-
-	return (myhostid);
+	return (spa->spa_hostid);
 }
 
 boolean_t