riscv: ftrace: Properly acquire text_mutex to fix a race condition

riscv: ftrace: Properly acquire text_mutex to fix a race condition

As reported by lockdep, some patching was done without acquiring
text_mutex, so there could be a race when mapping the page to patch
since we use the same fixmap entry.

Reported-by: Han Gao <rabenda.cn@gmail.com>
Reported-by: Vivian Wang <wangruikang@iscas.ac.cn>
Reported-by: Yao Zi <ziyao@disroot.org>
Closes: https://lore.kernel.org/linux-riscv/aGODMpq7TGINddzM@pie.lan/
Tested-by: Yao Zi <ziyao@disroot.org>
Tested-by: Han Gao <rabenda.cn@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250711-alex-fixes-v2-1-d85a5438da6c@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>