f2fs: Move err variable to function scope in f2fs_fill_dentries()

This is preparation for the following backported fixes.  It was done
upstream as part of commit e1293bdf

 "f2fs: plug readahead IO in
readdir()", the rest of which does not seem suitable for stable.

Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>