For the LKM_E_LOAD case of the DISPATCH() macro, use lkmexists() to
make sure we don't have another instance the module we're trying to
load already loaded _before_ calling the module's load() function.
If lkmexists() returns true, return EEXIST without trying to load
the module.
For most types of modules, the individual dispatch functions in the
kernel check for duplicated modules, but for LM_MISC we can't trust
the module to do the checks itself. Currently, the kernel does
do an lkmexists() check on LM_MISC modules, but not until after
the module's load() function has been called, which is too late
for it to do any good. If the load() function does irreversible
things to the kernel, the belated lkmexists() check forces an
unload() and a crash.
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