cifs: Replace a couple of one-element arrays with flexible-array members

One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element arrays with flexible-array
member in structs negotiate_req and extended_response, and refactor the
rest of the code, accordingly.

Also, make use of the DECLARE_FLEX_ARRAY() helper to declare flexible
array member EncryptionKey in union u. This new helper allows for
flexible-array members in unions.

Change pointer notation to proper array notation in a call to memcpy()
where flexible-array member DialectsArray is being used as destination
argument.

Important to mention is that doing a build before/after this patch results
in no binary output differences.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/229
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>