net/9p: show error message if user 'msize' cannot be satisfied

net/9p: show error message if user 'msize' cannot be satisfied

If user supplied a large value with the 'msize' option, then
client would silently limit that 'msize' value to the maximum
value supported by transport. That's a bit confusing for users
of not having any indication why the preferred 'msize' value
could not be satisfied.

Link: https://lkml.kernel.org/r/783ba37c1566dd715b9a67d437efa3b77e3cd1a7.1640870037.git.linux_oss@crudebyte.com
Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>