e2fsck: validate that fscrypt_symlink_data.len is not too large

e2fsck: validate that fscrypt_symlink_data.len is not too large

Make e2fsck start validating that the ciphertext length stored in the
header of an encrypted symlink target, plus the header itself, is no
larger than a filesystem block.  Previously e2fsck only verified that
this size is not exactly equal to a filesystem block.  This was
sufficient for unencrypted symlinks, where the "actual length" is
computed using strnlen(), but not for encrypted symlinks; the kernel
also considers encrypted symlinks with too-large ciphertext length to be
invalid.

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