fs: Mark function as static in ext3/dir.c

Mark function as static in ext3/dir.c because it is not used outside
this file.

This also eliminates the following warning in ext3/dir.c:
fs/ext3/dir.c:278:8: warning: no previous prototype for ‘ext3_dir_llseek’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Jan Kara <jack@suse.cz>