fix the PANIC: metaslab_free_dva(): bad DVA with zfs #3937

[hsepeng]

the panic was introduced by the following scenario:
in the previous transaction group, the bonus buffer
was entirely used to store the attributes for the
dnode which override the dn_spill field.
however, when adding more attributes to the file,
it will need the spill block to hold the extra
attributes overflowing the bonus buffer.
make sure to clear the garbage left in the dn_spill
field which was the previous attributes in bonus
buffer, otherwise, after writing out the spill block
data to the new allocated dva, it will try to free
the old block pointed by the invalid dn_spill, that
would introduce the panic

[hsepeng]

@behlendorf did the failure relate to my fix patch?

[behlendorf]

@hsepeng both kernel.org builders failed due to compatibility issues with the latest kernel.org kernel. These problems have been fixed in master if you rebase the PR. The other two failures look unrelated to your patch, but it would still be good to refresh the PR against master to resolve the kernel.org build issue.