Fix e2fsck's check_is_really_dir logic

Fix e2fsck's check_is_really_dir logic

Fix a typo which could cause e2fsck to throw an I/O error while doubling
checking whether or not a special device file was really an inode.
Also, don't do this tests on symbolic links since for filesystems with a
large numbers of symlinks it could degrade performance and increases the
risk for false positives.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>