pseries/papr-hvpipe: Fix the usage of copy_to_user()

pseries/papr-hvpipe: Fix the usage of copy_to_user()

copy_to_user() return bytes_not_copied to the user buffer. If there was
an error writing bytes into the user buffer, i.e. if copy_to_user
returns a non-zero value, then we should simply return -EFAULT from the
->read() call.

Otherwise, in the non-patched version, we may end up mixing
"bytes_not_copied + bytes_copied (HVPIPE_HDR_LEN)" as the return value
to the user in ->read() call

Also let's make sure we clear the hvpipe_status flag, if we have
consumed the hvpipe msg by making the rtas call. ret = -EFAULT means
copy_to_user has failed but that still means that the msg was read from
the hvpipe, hence for both cases, success & -EFAULT, we should clear the
HVPIPE_MSG_AVAILABLE flag in hvpipe_status.

Cc: stable@vger.kernel.org
Fixes: cebdb522fd3edd1 ("powerpc/pseries: Receive payload with ibm,receive-hvpipe-msg RTAS")
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/8fda3212a1ad48879c174e92f67472d9b9f1c3b7.1777606826.git.ritesh.list@gmail.com