staging: exfat: dedicate count_entries() to sub-dir counting.

count_entries() function is only used to count sub-dirs.
Clarify the role and rename to count_dir_entries().

Signed-off-by: Tetsuhiro Kohada <Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp>
Link: https://lore.kernel.org/r/20200217072941.34116-2-Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>