eCryptfs: Check Tag 11 literal data buffer size

Tag 11 packets are stored in the metadata section of an eCryptfs file to
store the key signature(s) used to encrypt the file encryption key.
After extracting the packet length field to determine the key signature
length, a check is not performed to see if the length would exceed the
key signature buffer size that was passed into parse_tag_11_packet().

Thanks to Ramon de Carvalho Valle for finding this bug using fsfuzzer.

Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Cc: stable@kernel.org (2.6.27 and 30)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>