e2fsck: require that fast symlinks don't have EXT4_EXTENTS_FL

e2fsck: require that fast symlinks don't have EXT4_EXTENTS_FL

It doesn't make sense for EXT4_EXTENTS_FL to be set on a fast symlink.
The kernel doesn't set it, and it ignores it if set.  Meanwhile, e2fsck
is stricter: it will try to validate the extent tree, which will almost
certainly fail (assuming the symlink is, in fact, a fast symlink).

Make this behavior more explicit by rejecting EXT4_EXTENTS_FL for fast
symlinks, rather than going ahead and trying to validate an extent tree.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>