Linux: fprobe: Terminate the fgraph_data list when the reservation is not filled

In the Linux kernel, the following vulnerability has been resolved:

fprobe: Terminate the fgraph_data list when the reservation is not filled

fprobe_fgraph_entry() reserves shadow stack space for every fprobe with an exit handler, but only fills it for those whose entry handler returns

1. fgraph_reserve_data() does not clear the area, so fprobe_return()

parses the unused tail as headers left over from an earlier call, and an exit handler can run twice or despite its entry handler asking to skip it.

Write a zero word after the last entry to terminate the walk. A zeroed slot does not decode to a NULL fprobe on the arches that encode the header into one unsigned long, since arch_decode_fprobe_header_fp() ORs in FPROBE_HEADER_MSB_PATTERN, so make read_fprobe_header() return NULL for a zeroed slot.