MFC r364027 by arichardson: Fix linker error in libuutil with recent LLVM
This also fixes nanobsd-style build (cross-compiling).
Original commit log:
Not marking the function as static can result in a linker error: undefined reference to __assfail [--no-allow-shlib-undefined] I noticed this error after updating our CHERI LLVM to the latest upstream LLVM HEAD revision. This change effectively reverts r329984 and marks dmu_buf_init_user as static (which keeps the GCC build happy).