9p: only copy valid iattrs in 9P2000.L setattr implementation

9p: only copy valid iattrs in 9P2000.L setattr implementation

The 9P2000.L setattr method v9fs_vfs_setattr_dotl() copies struct iattr
values without checking whether they are valid causing unitialized
values to be copied. The 9P2000 setattr method v9fs_vfs_setattr() method
gets this right. Check whether struct iattr fields are valid first
before copying in v9fs_vfs_setattr_dotl() too and make sure that all
other fields are set to 0 apart from {g,u}id which should be set to
INVALID_{G,U}ID. This ensure that they can be safely sent over the wire
or printed for debugging later on.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/000000000000a0d53f05d1c72a4c%40google.com
Cc: Eric Van Hensbergen <[email protected]>
Cc: Latchesar Ionkov <[email protected]>
Cc: Dominique Martinet <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reported-by: [email protected]
Signed-off-by: Christian Brauner <[email protected]>
[Dominique: do not set a/mtime with just ATTR_A/MTIME as discussed]
Signed-off-by: Dominique Martinet <[email protected]>