fuse: minor improvements for readdir(plus)

Instead of using 'int' for the sizes, use a 'size_t' as it is more
correct. Save the size of a fuse_dirent in a temporary variable so that
strlen() on the filename is called fewer times.

Also correcting some typos in comments.

Change-Id: Ic62d9d729a86a1a6a53ed1354fce153bac01d860
BUG: 1074023
Reported-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/7547
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>