MFC r364027 by arichardson: Fix linker error in libuutil with recent LLVM

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).

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