CVE-2026-68146: ftrace: Add global mutex to serialize trace_parser access

In the Linux kernel, the following vulnerability has been resolved:

ftrace: Add global mutex to serialize traceparser access

In ftrace, the traceparser structure is allocated and initialized when a trace file is opened, and is subsequently used across write and release handlers to parse user input.

The affected handler paths and their specific functions are: - Open paths: ftraceregexopen(), ftracegraphopen() - Write paths: ftraceregexwrite(), ftracegraphwrite() - Release paths: ftraceregexrelease(), ftracegraphrelease()

If userspace opens a trace file descriptor and shares it across multiple threads, concurrent write calls will race on the parser's internal state, specifically the 'idx', 'cont', and 'buffer' fields, leading to corrupted input or undefined behavior.

Fix this by adding a global mutex, parserlock, to serialize all access to traceparser across write and release paths, preventing concurrent corruption of parser state.