9pfs: T_readdir latency optimization

9pfs: T_readdir latency optimization

Make top half really top half and bottom half really bottom half:

Each T_readdir request handling is hopping between threads (main
I/O thread and background I/O driver threads) several times for
every individual directory entry, which sums up to huge latencies
for handling just a single T_readdir request.

Instead of doing that, collect now all required directory entries
(including all potentially required stat buffers for each entry) in
one rush on a background I/O thread from fs driver by calling the
previously added function v9fs_co_readdir_many() instead of
v9fs_co_readdir(), then assemble the entire resulting network
response message for the readdir request on main I/O thread. The
fs driver is still aborting the directory entry retrieval loop
(on the background I/O thread inside of v9fs_co_readdir_many())
as soon as it would exceed the client's requested maximum R_readdir
response size. So this will not introduce a performance penalty on
another end.

Also: No longer seek initial directory position in v9fs_readdir(),
as this is now handled (more consistently) by
v9fs_co_readdir_many() instead.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <c7c3d1cf4e86611538cef44897842819d9359d7a.1596012787.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>