tracing: Fix memory leak of iter->temp when reading trace_pipe

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

tracing: Fix memory leak of iter->temp when reading trace_pipe

kmemleak reports:

unreferenced object 0xffff88814d14e200 (size 256):

comm "cat", pid 336, jiffies 4294871818 (age 779.490s)

hex dump (first 32 bytes):

04 00 01 03 00 00 00 00 08 00 00 00 00 00 00 00 ................

0c d8 c8 9b ff ff ff ff 04 5a ca 9b ff ff ff ff .........Z......

backtrace:

[<ffffffff9bdff18f>] __kmalloc+0x4f/0x140

[<ffffffff9bc9238b>] trace_find_next_entry+0xbb/0x1d0

[<ffffffff9bc9caef>] trace_print_lat_context+0xaf/0x4e0

[<ffffffff9bc94490>] print_trace_line+0x3e0/0x950

[<ffffffff9bc95499>] tracing_read_pipe+0x2d9/0x5a0

[<ffffffff9bf03a43>] vfs_read+0x143/0x520

[<ffffffff9bf04c2d>] ksys_read+0xbd/0x160

[<ffffffff9d0f0edf>] do_syscall_64+0x3f/0x90

[<ffffffff9d2000aa>] entry_SYSCALL_64_after_hwframe+0x6e/0xd8

when reading file 'trace_pipe', 'iter->temp' is allocated or relocated

in trace_find_next_entry() but not freed before 'trace_pipe' is closed.

To fix it, free 'iter->temp' in tracing_release_pipe().