LU-1540 e2fsck: add missing symlink NUL terminator

LU-1540 e2fsck: add missing symlink NUL terminator

If a long symbolic link target is written into an external block
without a NUL terminator, its length is decided by the inode's size.
Make symlink check add a NUL termination in such cases if needed.

Such faulty symlinks were generated by osd-ldiskfs on the MDS until
Lustre 2.1.3 and Lustre 2.3.  The in-kernel code would handle such
unterminated symlinks correctly, since it used the inode size to
determine the symlink length, but e2fsck would assume the symlink
is broken if there wasn't a trailing NUL.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I4419b30f1adb4a7d273796a936427aa351510213