nvmet: pci-epf: reject too-short SGL segments

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

nvmet: pci-epf: reject too-short SGL segments

The host controls an SGL segment's length. A nonzero length smaller than
one SGL descriptor makes nr_descs zero, so
nvmet_pci_epf_get_sgl_segment() reads the type byte of sgls[-1] outside
the allocated buffer. If that byte contains a segment descriptor type,
the function also copies the preceding 16 bytes and returns a negative
descriptor count.

The read was reproduced in 3/3 KASAN boots with only the host transfer
mocked by a KUnit test:

BUG: KASAN: slab-out-of-bounds in nvmet_pci_epf_get_sgl_segment
Read of size 1 by task kunit_try_catch
Call Trace:
nvmet_pci_epf_get_sgl_segment
nvmet_pci_epf_short_sgl_test
Kernel panic - not syncing: KASAN: panic_on_warn set ...

Reject segments that cannot hold one descriptor before allocating or
transferring their contents.