powerpc/vas: Return -EINVAL if the offset is non-zero in mmap()

The user space calls mmap() to map VAS window paste address
and the kernel returns the complete mapped page for each
window. So return -EINVAL if non-zero is passed for offset
parameter to mmap().

See Documentation/arch/powerpc/vas-api.rst for mmap()
restrictions.

Co-developed-by: Jonathan Greental <yonatan02greental@gmail.com>
Signed-off-by: Jonathan Greental <yonatan02greental@gmail.com>
Reported-by: Jonathan Greental <yonatan02greental@gmail.com>
Fixes: dda44eb2 ("powerpc/vas: Add VAS user space API")
Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250610021227.361980-2-maddy@linux.ibm.com