memblock tests: Fix mutex related build error

Fix mutex and free_reserved_area() related build errors which have
been introduced by commit 74e2498c ("mm/memblock: Add reserved
memory release function").

Fixes: 74e2498c ("mm/memblock: Add reserved memory release function")
Reported-by: Wei Yang <richard.weiyang@gmail.com>
Closes: https://lore.kernel.org/all/20250405023018.g2ae52nrz2757b3n@master/


Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/174399023133.47537.7375975856054461445.stgit@devnote2


Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>