rxrpc: Truncate UTS_RELEASE for rxrpc version

UTS_RELEASE has a maximum length of 64 which can cause rxrpc_version to
exceed the 65 byte message limit.

Per the rx spec[1]: "If a server receives a packet with a type value of 13,
and the client-initiated flag set, it should respond with a 65-byte payload
containing a string that identifies the version of AFS software it is
running."

The current implementation causes a compile error when WERROR is turned on
and/or UTS_RELEASE exceeds the length of 49 (making the version string more
than 64 characters).

Fix this by generating the string during module initialisation and limiting
the UTS_RELEASE segment of the string does not exceed 49 chars.  We need to
make sure that the 64 bytes includes "linux-" at the front and " AF_RXRPC"
at the back as this may be used in pattern matching.

Fixes: 44ba0698

 ("RxRPC: Handle VERSION Rx protocol packets")
Reported-by: Kenny Ho <Kenny.Ho@amd.com>
Link: https://lore.kernel.org/r/20230523223944.691076-1-Kenny.Ho@amd.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Kenny Ho <Kenny.Ho@amd.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Andrew Lunn <andrew@lunn.ch>
cc: David Laight <David.Laight@ACULAB.COM>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
Link: https://web.mit.edu/kolya/afs/rx/rx-spec [1]
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Link: https://lore.kernel.org/r/654974.1685100894@warthog.procyon.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>