9p: add P9_ERRMAX for 9p2000 and 9p2000.u

Add P9_ERRMAX macro to 9P protocol header which reflects the maximum
error string length of Rerror replies for 9p2000 and 9p2000.u protocol
versions. Unfortunately a maximum error string length is not defined by
the 9p2000 spec, picking 128 as value for now, as this seems to be a
common max. size for POSIX error strings in practice.

9p2000.L protocol version uses Rlerror replies instead which does not
contain an error string.

Link: https://lkml.kernel.org/r/3f23191d21032e7c14852b1e1a4ae26417a36739.1657920926.git.linux_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

include/net/9p/9p.h

@@ -331,6 +331,9 @@ enum p9_qid_t {
 /* size of header for zero copy read/write */
 #define P9_ZC_HDR_SZ 4096
 
+/* maximum length of an error string */
+#define P9_ERRMAX 128
+
 /**
  * struct p9_qid - file system entity information
  * @type: 8-bit type &p9_qid_t