ksmbd: fix Out-of-Bounds Read in ksmbd_vfs_stream_read

In the Linux kernel, the following vulnerability has been resolved: ksmbd: fix Out-of-Bounds Read in ksmbd_vfs_stream_read An offset from client could be a negative value, It could lead to an out-of-bounds read from the stream_buf. Note that this issue is coming when setting 'vfs objects = streams_xattr parameter' in ksmbd.conf.