sys/geom: use proper style for sizeof operator

No functional change is intended.

Missing parentheses around sizeof operands have been added with
a coccinnele patch:

@disable paren@
expression E;
@@
(
sizeof(E)
|
sizeof
+(
E
+)
)

Spaces between sizeof and a parenthesis have been removed with sed.

Discussed with: imp

(cherry picked from commit 150834f8fa57c1ee42e4dd5a42e7faf08ec5b2e5)