util/log: Limit RCUCloseFILE to file closing

util/log: Limit RCUCloseFILE to file closing

Use FILE* for global_file.  We can perform an rcu_read on that
just as easily as RCUCloseFILE*.  This simplifies a couple of
places, where previously we required taking the rcu_read_lock
simply to avoid racing to dereference RCUCloseFile->fd.

Only allocate the RCUCloseFile prior to call_rcu.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-39-richard.henderson@linaro.org>