9pfs: differentiate readdir lock between 9P2000.u vs. 9P2000.L

9pfs: differentiate readdir lock between 9P2000.u vs. 9P2000.L

Previous patch suggests that it might make sense to use a different mutex
type now while handling readdir requests, depending on the precise
protocol variant, as v9fs_do_readdir_with_stat() (used by 9P2000.u) uses
a CoMutex to avoid deadlocks that might happen with QemuMutex otherwise,
whereas do_readdir_many() (used by 9P2000.L) should better use a
QemuMutex, as the precise behaviour of a failed CoMutex lock on fs driver
side would not be clear.

And to avoid the wrong lock type being used, be now strict and error out
if a 9P2000.L client sends a Tread on a directory, and likeweise error out
if a 9P2000.u client sends a Treaddir request.

This patch is just intended as transitional measure, as currently 9P2000.u
vs. 9P2000.L implementations currently differ where the main logic of
fetching directory entries is located at (9P2000.u still being more top
half focused, while 9P2000.L already being bottom half focused in regards
to fetching directory entries that is).

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <9a2ddc347e533b0d801866afd9dfac853d2d4106.1596012787.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>