GFS2: Remove dirent_first() function

This function only had one caller left, and that caller only
called it for leaf blocks, hence one branch of the "if" was
never taken. In addition the call to get_left had already
verified the metadata type, so the function can be reduced
to a single line of code in its caller.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>