eCryptfs: check readlink result was not an error before using it

eCryptfs: check readlink result was not an error before using it

The result from readlink is being used to index into the link name
buffer without checking whether it is a valid length. If readlink
returns an error this will fault or cause memory corruption.

Cc: Tyler Hicks <[email protected]>
Cc: Dustin Kirkland <[email protected]>
Cc: [email protected]
Signed-off-by: Duane Griffin <[email protected]>
Acked-by: Michael Halcrow <[email protected]>
Acked-by: Tyler Hicks <[email protected]>
Signed-off-by: Al Viro <[email protected]>