Syscall wrapper for fcntl() is missing checks on flock structure.

The following fcntl commands take 'struct flock' or 'struct flock64' as ARG3:
-   F_GETLK, F_SETLK, VKI_F_SETLKW, VKI_F_OFD_GETLK, VKI_F_OFD_SETLK,
    F_OFD_SETLKW:
- VKI_F_GETLK64, VKI_F_SETLK64, VKI_F_SETLKW64

The kernel fcntl implementation always reads the following fields of flock or flock64: l_type, l_whence, l_start, l_len.
In case of F_GETLK, F_OFD_GETLK, and F_GETLK64, field l_pid is going to be written.

Therefore corresponding pre-field-read and post-field-write statements need to be in place.