Fix readdir() bug when a non-zero offset is specified in filler (#269)

Fix readdir() bug when a non-zero offset is specified in filler (#269)

The bug occurs when a filesystem client reads a directory until the end,
seeks using seekdir() to some valid non-zero position and calls
readdir(). A valid 'struct dirent *' is expected, but NULL is returned
instead. Pseudocode demonstrating the bug:

DIR *dp = opendir("some_dir");
struct dirent *de = readdir(dp);

/* Get offset of the second entry */
long offset = telldir(dp);

/* Read directory until the end */
while (de)
	de = readdir(de);

seekdir(dp, offset);
de = readdir(dp);
/* de must contain the second entry, but NULL is returned instead */

The reason of the bug is that when the end of directory is reached, the
kernel calls FUSE_READDIR op with an offset at the end of directory, so
the filesystem's .readdir callback never calls the filler function, and
we end up with dh->filled set to 1. After seekdir(), FUSE_READDIR is
called again with a new offset, but this time the filesystem's .readdir
callback is never called, and an empty reply is returned.

Fix by setting dh->filled to 1 only when zero offsets are given to
filler function.