Add mutex_enter_interruptible() for interruptible sleeping IOCTLs

Many long-running ZFS ioctls lock the spa_namespace_lock, forcing
concurrent ioctls to sleep for the mutex. Previously, the only
option is to call mutex_enter() which sleeps uninterruptibly. This
is a usability issue for sysadmins, for example, if the admin runs
`zpool status` while a slow `zpool import` is ongoing, the admin's
shell will be locked in uninterruptible sleep for a long time.

This patch resolves this admin usability issue by introducing
mutex_enter_interruptible() which sleeps interruptibly while waiting
to acquire a lock. It is implemented for both Linux and FreeBSD.

The ZFS_IOC_POOL_CONFIGS ioctl, used by `zpool status`, is changed to
use this new macro so that the command can be interrupted if it is
issued during a concurrent `zpool import` (or other long-running
operation).

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Thomas Bertschinger <bertschinger@lanl.gov>
Closes #15360

module/zfs/zfs_ioctl.c

@@ -1582,8 +1582,9 @@ zfs_ioc_pool_configs(zfs_cmd_t *zc)
 	nvlist_t *configs;
 	int error;
 
-	if ((configs = spa_all_configs(&zc->zc_cookie)) == NULL)
-		return (SET_ERROR(EEXIST));
+	error = spa_all_configs(&zc->zc_cookie, &configs);
+	if (error)
+		return (error);
 
 	error = put_nvlist(zc, configs);