9p: darwin: Move XATTR_SIZE_MAX->P9_XATTR_SIZE_MAX

9p: darwin: Move XATTR_SIZE_MAX->P9_XATTR_SIZE_MAX

Signed-off-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Michael Roitzsch <reactorcontrol@icloud.com>
Because XATTR_SIZE_MAX is not defined on Darwin,
create a cross-platform P9_XATTR_SIZE_MAX instead.

[Will Cohen: - Adjust coding style
             - Lower XATTR_SIZE_MAX to 64k
             - Add explanatory context related to XATTR_SIZE_MAX]
[Fabian Franz: - Move XATTR_SIZE_MAX reference from 9p.c to
                 P9_XATTR_SIZE_MAX in 9p.h]
Signed-off-by: Will Cohen <wwcohen@gmail.com>
Signed-off-by: Fabian Franz <fabianfranz.oss@gmail.com>
[Will Cohen: - For P9_XATTR_MAX, ensure that Linux uses
               XATTR_SIZE_MAX, Darwin uses 64k, and error
               out for undefined hosts]
Signed-off-by: Will Cohen <wwcohen@gmail.com>
Message-Id: <20220227223522.91937-7-wwcohen@gmail.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>