ksmbd: prevent out-of-bounds stream writes by validating *pos

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: prevent out-of-bounds stream writes by validating *pos

ksmbd_vfs_stream_write() did not validate whether the write offset (*pos) was within the bounds of the existing stream data length (v_len). If *pos was greater than or equal to v_len, this could lead to an out-of-bounds memory write.

This patch adds a check to ensure *pos is less than v_len before proceeding. If the condition fails, -EINVAL is returned.