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The kernel does indeed support loading modules at runtime.

These modules are not libraries - the rest of the kernel does not depend on the code in the modules. Rather it is the other way around. The modules depend on the rest of the code in the kernel.

Since they connect to the rest of the kernel via a narrow API, and since the kernel was not written with specific awareness of or dependency on or derivation from any such module, therefore, if the actual code in the module was not derived from Linux kernel code, then the module is not subject to the GPL license of the kernel, because the module is a separate work.

Libraries are used by what links to them; modules use what they link to. Kernel loadable modules are not libraries, not technically, and not in common terminology.

FYI, what I had in mind is this disclaimer at the top of the Linux "COPYING" file before the GPL text, which clarifies Linus Torvalds interpretation of the license. I believe his interpretation differs from Stallman's, thus his rejection of later versions of the GPL.

NOTE! This copyright does *not* cover user programs that use kernel services by normal system calls - this is merely considered normal use of the kernel, and does *not* fall under the heading of "derived work". Also note that the GPL below is copyrighted by the Free Software Foundation, but the instance of code that it refers to (the Linux kernel) is copyrighted by me and others who actually wrote it.
Also note that the only valid version of the GPL as far as the kernel is concerned is _this_ particular version of the license (ie v2, not v2.2 or v3.x or whatever), unless explicitly otherwise stated.