io_uring/rsrc: reject zero-length fixed buffer import

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

io_uring/rsrc: reject zero-length fixed buffer import

validate_fixed_range() admits buf_addr at the exact end of the
registered region when len is zero, because the check uses strict
greater-than (buf_end > imu->ubuf + imu->len). io_import_fixed()
then computes offset == imu->len, which causes the bvec skip logic
to advance past the last bio_vec entry and read bv_offset from
out-of-bounds slab memory.

Return early from io_import_fixed() when len is zero. A zero-length
import has no data to transfer and should not walk the bvec array
at all.

BUG: KASAN: slab-out-of-bounds in io_import_reg_buf+0x697/0x7f0
Read of size 4 at addr ffff888002bcc254 by task poc/103
Call Trace:
io_import_reg_buf+0x697/0x7f0
io_write_fixed+0xd9/0x250
__io_issue_sqe+0xad/0x710
io_issue_sqe+0x7d/0x1100
io_submit_sqes+0x86a/0x23c0
__do_sys_io_uring_enter+0xa98/0x1590
Allocated by task 103:
The buggy address is located 12 bytes to the right of
allocated 584-byte region [ffff888002bcc000, ffff888002bcc248)