In dbuf_dirty(), we need to grab the dn_struct_rwlock before looking at the
db_blkptr, to prevent it from being changed by syncing context.
Otherwise we may see that ztest got a segfault from this stack:
libzpool.so.1`dva_get_dsize_sync+0x98(872f000, b32b240, fed7811b, 0, b4cda20, 0)
libzpool.so.1`bp_get_dsize+0x60(872f000, b32b240, 0, 97cb780, 9d4c1a8, 0)
libzpool.so.1`dbuf_dirty+0x9b3(ce0a100, 97cb780, 9, fecd2530)
libzpool.so.1`dmu_buf_will_dirty+0xc3(ce0a100, 97cb780, ea293d6c, 1)
libzpool.so.1`zap_lockdir+0x1a0(8aaa3c0, 1, 0, 97cb780, 1, 1)
libzpool.so.1`zap_remove_norm+0x30(8aaa3c0, 1, 0, 8728b10, 0, 97cb780)
libzpool.so.1`zap_remove+0x29(8aaa3c0, 1, 0, 8728b10, 97cb780, a)
ztest_replay_remove+0x225(ea294588, 8728ae8, 0, 38010000, 0, 0)
ztest_remove+0x9f(ea294588, ea293f50, 4, 3)
ztest_object_init+0x78(ea294588, ea293f50, 4e0, 1)
ztest_dmu_object_alloc_free+0x71(ea294588, 13)
ztest_dmu_objset_create_destroy+0x224(80cef08, 13, 0, 805d36c, 9017ad44, 0)
ztest_execute+0x89(a, 807c720, 13, 0)
ztest_thread+0xea(13, 0, 0, 0)
libc.so.1`_thrp_setup+0x88(f0983240)
libc.so.1`_lwp_start(f0983240, 0, 0, 0, 0, 0)
Looking into it a bit, we see that this is an embedded blockpointer, so
BP_GET_NDVAS should have returned 0:
     b32b240::blkptr
EMBEDDED [L0 ZAP_OTHER] et=0 LZ4 size=200L/4aP birth=80L
Instead, it looks like another thread is modifying this blockpointer:
     b32b240::ugrep | ::whatis
f47a0e0c is in [ stack tid=0x19f ]
ebd6ec40 is in [ stack tid=0x226 ]
ea293bd0 is in [ stack tid=0x244 ]
ea293be4 is in [ stack tid=0x244 ]