Verify every block pointer is either embedded, hole, or has a valid DVA

Now instead of crashing when attempting to read the corrupt block
pointer, ZFS will return ECKSUM, in a stack that looks like this:

none:set-error
zfs.ko`arc_read+0x1d82
zfs.ko`dbuf_read+0xa8c
zfs.ko`dmu_buf_hold_array_by_dnode+0x292
zfs.ko`dmu_read_uio_dnode+0x47
zfs.ko`zfs_read+0x2d5
zfs.ko`zfs_freebsd_read+0x7b
kernel`VOP_READ_APV+0xd0
kernel`vn_read+0x20e
kernel`vn_io_fault_doio+0x45
kernel`vn_io_fault1+0x15e
kernel`vn_io_fault+0x150
kernel`dofileread+0x80
kernel`sys_read+0xb7
kernel`amd64_syscall+0x424
kernel`0xffffffff810633cb

This patch should hopefully also prevent such corrupt block pointers
from being written to disk in the first place.

And in zdb, don't crash when printing a block pointer with no valid
DVAs. If a block pointer isn't embedded yet doesn't have any valid
DVAs, that's a data corruption bug. zdb should be able to handle the
situation gracefully.

Finally, remove an extra check for gang blocks in SNPRINTF_BLKPTR. This
check, which compares the asizes of two different DVAs within the same
BP, was added by illumos-gate commit b24ab67[^1], and I can't understand
why. It doesn't appear to do anything useful, so remove it.

[^1]: https://github.com/illumos/illumos-gate/commit/b24ab6762772a3f6a89393947930c7fa61306783

Fixes #17077
Sponsored by: ConnectWise
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Alek Pinchuk <pinchuk.alek@gmail.com>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #17078