swtpm: cuse: Extend usage of FILE_OPS_LOCK to protect a reading thread

swtpm: cuse: Extend usage of FILE_OPS_LOCK to protect a reading thread

Extend usage of the FILE_OPS_LOCK to prevent other threads from reading or
writing commands or doing ioctls while the current thread is reading a
response. This prevents a race condition where ptm_read_offset is set to 0
by a thread writing a new command to the device while the current thread
is reading a response from the device and needs this offset.

Resolves: https://github.com/stefanberger/swtpm/issues/725
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>