filedescriptor into it. Make sure that td_fpop is NULL when calling
d_mmap from dev_pager_getpages().
Change guards against td_fpop field being non-NULL with private state
for another device, and against sudden clearing the td_fpop. This
could occur when either a driver method calls another driver through
the filedescriptor operation, or a page fault happen while driver is
writing to a memory backed by another driver.
Noted by: rwatson
Tested by: rnoland
MFC after: 3 days
7818e0a545