tst_fs_has_free: Use f_bavail instead of f_bfree.

The f_bfree describes total number of free blocks while f_bavail
describes number of free blocks available to unprivileged user.

So if tests checks for available size and the code that creates the file
runs as unprivileged user it may have still fail even after this check
promised us enough disk space.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>