Prevent scrubbing a read-only pool

While it would be nice to be able to scrub a pool imported read-only
this will currently trip an ASSERT.  Before we can support this there
are some designs challenges which need to be thought through first.

For starters, a read-only import skips reading certain information 
from disk which it knows won't be needed, such as the space maps.
Furthermore, the scrub process expects to be checkpoint it's progress, 
update the on disk error log, and issue repair IO.  None of which 
would be possible when the pool is imported read-only.  

Each of these wrinkles can certainly be handled, but that will take 
some signifcant work.  In the meanwhile we disable the 'zpool scrub' 
command when the pool is imported read-only.

Reviewed-by: Alan Somers <asomers@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #17527
Closes #17717

Author: behlendorf

module/zfs/zfs_ioctl.c

@@ -7619,7 +7619,7 @@ zfs_ioctl_init(void)
 
 	zfs_ioctl_register("scrub", ZFS_IOC_POOL_SCRUB,
 	    zfs_ioc_pool_scrub, zfs_secpolicy_config, POOL_NAME,
-	    POOL_CHECK_NONE, B_TRUE, B_TRUE,
+	    POOL_CHECK_SUSPENDED | POOL_CHECK_READONLY, B_TRUE, B_TRUE,
 	    zfs_keys_pool_scrub, ARRAY_SIZE(zfs_keys_pool_scrub));
 
 	zfs_ioctl_register("get_props", ZFS_IOC_POOL_GET_PROPS,