trace: f2fs: Fix kasan slab-out-of-bounds

This patch fixes the following KASAN report:

KASAN: slab-out-of-bounds in string+0xb4/0x120
Read of size 1 at addr fffffffbf2f3b510 by task atrace/9219

Call trace:
dump_backtrace+0x0/0x3e8
show_stack+0x20/0x30
dump_stack+0x13c/0x198
print_address_description+0x80/0x2e0
kasan_report_error+0x1ac/0x210
__asan_report_load2_noabort+0x0/0xb8
string+0xb4/0x120
vsnprintf+0x5f4/0x1658
seq_buf_vprintf+0x11c/0x200
trace_seq_printf+0x1a8/0x320
trace_raw_output_f2fs_lookup_start+0xe8/0x158
print_trace_line+0x3cc/0xeb0
s_show+0x154/0x248
seq_read+0x8a8/0x1378
__vfs_read+0xfc/0x550
vfs_read+0x108/0x268
ksys_read+0x124/0x2b0

Correctly assign name string for f2fs trace.

Change-Id: I8ed0cf7c542ead06479fb4fde65b01b5e739e6db
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>