seq_buf: Fix overflow in seq_buf_putmem_hex()

There's two variables being increased in that loop (i and j), and i
follows the raw data, and j follows what is being written into the buffer.
We should compare 'i' to MAX_MEMHEX_BYTES or compare 'j' to HEX_CHARS.
Otherwise, if 'j' goes bigger than HEX_CHARS, it will overflow the
destination buffer.

Link: https://lore.kernel.org/lkml/20210625122453.5e2fe304@oasis.local.home/
Link: https://lkml.kernel.org/r/20210626032156.47889-1-yun.zhou@windriver.com

Cc: stable@vger.kernel.org
Fixes: 5e3ca0ec76fce ("ftrace: introduce the "hex" output method")
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
