Fix panic when resizing ZFS zvol's

Resizing a ZFS ZVOL with debug enabled would result in a panic due to
recursion on dp_config_rwlock.

The upstream change "3464 zfs synctask code needs restructuring" changed
zvol_set_volsize to avoid the recursion on dp_config_rwlock, but this was
missed when originally merged in by r248571 due to significant differences
in our codebases in this area.

These changes also relied on bring in changes from upstream:
3557 dumpvp_size is not updated correctly when a dump zvol's size is
changed, which where also not present.

In order to help prevent future issues in this area a direct comparison
and diff minimisation from current upstream version (b515258) of zvol.c.

Differential Revision:	https://reviews.freebsd.org/D1302
MFC after:	1 month
X-MFC-With:	r276063 & r276066
Sponsored by:	Multiplay