smb: convert to ctime accessor functions

In later patches, we're going to change how the inode's ctime field is
used. Switch to using accessor functions instead of raw accesses of
inode->i_ctime.

Acked-by: Tom Talpey <tom@talpey.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Steve French <stfrench@microsoft.com>
Message-Id: <20230705190309.579783-72-jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>