Keep dirhash tailq locked throughout the entirety of ufsdirhash_destroy() to fix a potential race pointed out by pjd. Also use TAILQ_FOREACH_SAFE to iterate over dirhashes in ufsdirhash_lowmem(), so that we can continue iterating even after a dirhash is destroyed.
Keep dirhash tailq locked throughout the entirety of ufsdirhash_destroy() to fix
a potential race pointed out by pjd. Also use TAILQ_FOREACH_SAFE to iterate over
dirhashes in ufsdirhash_lowmem(), so that we can continue iterating even after a
dirhash is destroyed.

Suggested by:	pjd
Tested by:      pho
Approved by:	dwmalone (mentor)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase