ocfs2: Don't walk off the end of fast symlinks.

ocfs2 fast symlinks are NUL terminated strings stored inline in the
inode data area.  However, disk corruption or a local attacker could, in
theory, remove that NUL.  Because we're using strlen() (my fault,
introduced in a731d1 when removing vfs_follow_link()), we could walk off
the end of that string.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Cc: stable@kernel.org