In order to use `ena_global_lock` in sysctl context, it must be kept
outside the driver instance's software context, as sysctls can be called
before attach and after detach, leading to lock use before sx_init and
after sx_destroy otherwise.
Solve this issue by turning `ena_global_lock` into a file scope
variable, shared between all instances of the driver and associated
sysctl context, and in turn initialized/destroyed in dedicated
SYSINIT/SYSUNINIT functions.
As a side effect, this change also fixes existing race in the reset
routine, when simultaneously accessing sysctl exposed properties.
Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.
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