smb: smbdirect: provide explicit prototypes for cross .c file functions

smb: smbdirect: provide explicit prototypes for cross .c file functions

These prototypes are used between private .c files,
when they will be compiled alone into smbdirect.ko.

For now this uses the SMBDIRECT_USE_INLINE_C_FILES code path
and marks all function as '__maybe_unused static',
but this will make further changes easier.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>