Fix linker error in libuutil with recent LLVM

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).

Reviewed By: ZFS, asomers, freqlabs, mav
Differential Revision: https://reviews.freebsd.org/D25663