iowatcher: Fix buffer overwrite issue

iowatcher: Fix buffer overwrite issue

Current code allocates buffer for path based on strdup, which would let
the size of path equals to the size of blktrace_dest_dir. But the code
next that joins it with the filename of dump file, which would overwrite
the buffer, and triggered an issue like following:

$ ./iowatcher -t trace.dump -o trace.svg
Unable to find trace file ./trace.dumpY
                                      ^

Refactoring join_path a bit to fix this issue.

Cc: Liu Bo <liub.liubo@gmail.com>
Signed-off-by: Yuanhan Liu <yliu.null@gmail.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>