Verify dRAID empty sectors

Verify that all empty sectors are zero filled before using them to
calculate parity.  Failure to do so can result in incorrect parity
columns being generated and written to disk if the contents of an
empty sector are non-zero.  This was possible because the checksum
only protects the data portions of the buffer, not the empty sector
padding.

This issue has been addressed by updating raidz_parity_verify() to
check that all dRAID empty sectors are zero filled.  Any sectors
which are non-zero will be fixed, repair IO issued, and a checksum
error logged.  They can then be safely used to verify the parity.

This specific type of damage is unlikely to occur since it requires
a disk to have silently returned bad data, for an empty sector, while
performing a scrub.  However, if a pool were to have been damaged
in this way, scrubbing the pool with this change applied will repair
both the empty sector and parity columns as long as the data checksum
is valid.  Checksum errors will be reported in the `zpool status`
output for any repairs which are made.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12857

include/sys/vdev_draid.h

@@ -96,6 +96,7 @@ extern boolean_t vdev_draid_readable(vdev_t *, uint64_t);
 extern boolean_t vdev_draid_missing(vdev_t *, uint64_t, uint64_t, uint64_t);
 extern uint64_t vdev_draid_asize_to_psize(vdev_t *, uint64_t);
 extern void vdev_draid_map_alloc_empty(zio_t *, struct raidz_row *);
+extern int vdev_draid_map_verify_empty(zio_t *, struct raidz_row *);
 extern nvlist_t *vdev_draid_read_config_spare(vdev_t *);
 
 /* Functions for dRAID distributed spares. */

include/sys/vdev_raidz.h

@@ -32,6 +32,7 @@ extern "C" {
 #endif
 
 struct zio;
+struct raidz_col;
 struct raidz_row;
 struct raidz_map;
 #if !defined(_KERNEL)
@@ -49,6 +50,7 @@ void vdev_raidz_generate_parity(struct raidz_map *);
 void vdev_raidz_reconstruct(struct raidz_map *, const int *, int);
 void vdev_raidz_child_done(zio_t *);
 void vdev_raidz_io_done(zio_t *);
+void vdev_raidz_checksum_error(zio_t *, struct raidz_col *, abd_t *);
 
 extern const zio_vsd_ops_t vdev_raidz_vsd_ops;