Linux: Fix enum fsconfig_command detection in <sys/mount.h>
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 774058d7
("linux: Fix sys/mount.h usage with kernel headers").
(cherry picked from commit 2955ef4b)