BACKPORT: udf: Verify symlink size before loading it

UDF specification allows arbitrarily large symlinks. However we support
only symlinks at most one block large. Check the length of the symlink
so that we don't access memory beyond end of the symlink block.

Upstream commit: a1d47b262952a45aae62bd49cfaf33dd76c11a2c
CC: stable@vger.kernel.org
Reported-by: Carl Henrik Lunde <chlunde@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>