The #ifdef FSOPEN_CLOEXEC check did not work because the macro
was always defined in this header prior to the check, so that
the <linux/mount.h> contents did not matter.
Fixes commit 774058d72942249f71d74e7f2b639f77184160a6
("linux: Fix sys/mount.h usage with kernel headers").
(cherry picked from commit 2955ef4b7c9b56fcd7abfeddef7ee83c60abff98)