Stopping mysql server could result in an entry in mysql error
file: "InnoDB: Error: MySQL is freeing a thd".

This happened because InnoDB assumes that the server will never
call external_lock(F_UNLCK) in case external_lock(READ/WRITE)
failed.

Prior to this patch we haven't had strict definition whether
external_lock(F_UNLCK) must be called in case external_lock(READ/WRITE)
fails.

This patch states that we never call external_lock(F_UNLCK) in case
external_lock(READ/WRITE) fails.