bpf: Fix the test_task_vma selftest to support output shorter than 1 kB

The test for bpf_iter_task_vma assumes that the output will be longer
than 1 kB, as the comment above the loop says. Due to this assumption,
the loop becomes infinite if the output turns to be shorter than 1 kB.
The return value of read_fd_into_buffer is 0 when the end of file was
reached, and len isn't being increased any more.

This commit adds a break on EOF to handle short output correctly. For
the reference, this is the contents that I get when running test_progs
under vmtest.sh, and it's shorter than 1 kB:

00400000-00401000 r--p 00000000 fe:00 25867     /root/bpf/test_progs
00401000-00674000 r-xp 00001000 fe:00 25867     /root/bpf/test_progs
00674000-0095f000 r--p 00274000 fe:00 25867     /root/bpf/test_progs
0095f000-00983000 r--p 0055e000 fe:00 25867     /root/bpf/test_progs
00983000-00a8a000 rw-p 00582000 fe:00 25867     /root/bpf/test_progs
00a8a000-0484e000 rw-p 00000000 00:00 0
7f6c64000000-7f6c64021000 rw-p 00000000 00:00 0
7f6c64021000-7f6c68000000 ---p 00000000 00:00 0
7f6c6ac8f000-7f6c6ac90000 r--s 00000000 00:0d 8032
anon_inode:bpf-map
7f6c6ac90000-7f6c6ac91000 ---p 00000000 00:00 0
7f6c6ac91000-7f6c6b491000 rw-p 00000000 00:00 0
7f6c6b491000-7f6c6b492000 r--s 00000000 00:0d 8032
anon_inode:bpf-map
7f6c6b492000-7f6c6b493000 rw-s 00000000 00:0d 8032
anon_inode:bpf-map
7ffc1e23d000-7ffc1e25e000 rw-p 00000000 00:00 0
7ffc1e3b8000-7ffc1e3bc000 r--p 00000000 00:00 0
7ffc1e3bc000-7ffc1e3bd000 r-xp 00000000 00:00 0
7fffffffe000-7ffffffff000 --xp 00000000 00:00 0

Fixes: e8168840 ("selftests/bpf: Add test for bpf_iter_task_vma")
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211130181811.594220-1-maximmi@nvidia.com