barectf_platform_linux_fs_init(): accept DS file path, not trace dir.

barectf_platform_linux_fs_init(): accept DS file path, not trace dir.

This patch changes the Linux FS platform's
barectf_platform_linux_fs_init() function so that it accepts the data
stream file path instead of its trace directory.

This makes it possible to initialize many Linux FS platform contexts for
the same trace, each one targeting a distinct data stream file.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>