ipv6/esp6: Remove structure variables and alignment statements

The definition of this variable is just to find the length of the
structure after aligning the structure. The PTR alignment function
is to optimize the size of the structure. In fact, it doesn't seem
to be of much use, because both members of the structure are of
type u32.
So I think that the definition of the variable and the
corresponding alignment can be deleted, the value of extralen can
be directly passed in the size of the structure.

The clang_analyzer complains as follows:

net/ipv6/esp6.c:117:27 warning:

Value stored to 'extra' during its initialization is never read

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>