CVE-2024-26818 tools/rtla: Fix clang warning about mount_point var size

In the Linux kernel, the following vulnerability has been resolved: tools/rtla: Fix clang warning about mountpoint var size clang is reporting this warning: $ make HOSTCC=clang CC=clang LLVMIAS=1 ... clang -O -g -DVERSION="6.8.0-rc3" -flto=auto -fexceptions -fstack-protector-strong...