jffs2: validate symlink size in jffs2_do_read_inode_internal()

`csize' is read from disk and thus needs validation.  Otherwise a bogus
value 0xffffffff would turn the subsequent kmalloc(csize + 1, ...) into
kmalloc(0, ...), leading to out-of-bounds write.

This patch limits `csize' to JFFS2_MAX_NAME_LEN, which is also used
in jffs2_symlink().

Artem: we actually validate csize by checking CRC, so this 0xFFs cannot
come from empty flash region. But I guess an attacker could feed JFFS2
an image with random csize value, including 0xFFs.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>