OpenZFS 8126 - ztest assertion failed in dbuf_dirty due to dn_nlevels changing

OpenZFS 8126 - ztest assertion failed in dbuf_dirty due to dn_nlevels changing

The sync thread is concurrently modifying dn_phys->dn_nlevels
while dbuf_dirty() is trying to assert something about it, without
holding the necessary lock. We need to move this assertion further down
in the function, after we have acquired the dn_struct_rwlock.

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8126
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/0ef125d
Closes #6314