Speed up WB_SYNC_NONE when a WB_SYNC_ALL occurs simultaneously

Page writebacks with WB_SYNC_NONE can take several seconds to complete 
since they wait for the transaction group to close before being 
committed. This is usually not a problem since the caller does not 
need to wait. However, if we're simultaneously doing a writeback 
with WB_SYNC_ALL (e.g via msync), the latter can block for several 
seconds (up to zfs_txg_timeout) due to the active WB_SYNC_NONE 
writeback since it needs to wait for the transaction to complete 
and the PG_writeback bit to be cleared.

This commit deals with 2 cases:

- No page writeback is active. A WB_SYNC_ALL page writeback starts 
  and even completes. But when it's about to check if the PG_writeback 
  bit has been cleared, another writeback with WB_SYNC_NONE starts. 
  The sync page writeback ends up waiting for the non-sync page 
  writeback to complete.

- A page writeback with WB_SYNC_NONE is already active when a 
  WB_SYNC_ALL writeback starts. The WB_SYNC_ALL writeback ends up 
  waiting for the WB_SYNC_NONE writeback.

The fix works by carefully keeping track of active sync/non-sync 
writebacks and committing when beneficial.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shaan Nobee <sniper111@gmail.com>
Closes #12662
Closes #12790

tests/zfs-tests/cmd/Makefile.am

@@ -80,9 +80,10 @@ mkfiles_SOURCES = mkfiles.c
 mktree_SOURCES = mktree.c
 
 
-pkgexec_PROGRAMS += mmap_exec mmap_seek mmapwrite readmmap
+pkgexec_PROGRAMS += mmap_exec mmap_seek mmap_sync mmapwrite readmmap
 mmap_exec_SOURCES = mmap_exec.c
 mmap_seek_SOURCES = mmap_seek.c
+mmap_sync_SOURCES = mmap_sync.c
 mmapwrite_SOURCES = mmapwrite.c
 mmapwrite_LDADD = -lpthread
 readmmap_SOURCES = readmmap.c