smb: client: fix potential OOB read in smb3_enum_snapshots()

If snapshot_array_size is smaller than GMT_TOKEN_SIZE,
smb3_enum_snapshots() sets ret_data_len to
sizeof(struct smb_snapshot_array) without verifying the actual length
of the server's reply.

Because SMB2_ioctl() places no lower bound on the server-supplied
OutputCount and allocates retbuf to exactly that length, a short reply
results in ret_data_len exceeding the size of retbuf. The subsequent
copy_to_user() then reads past the end of retbuf, leaking adjacent slab
memory to userspace.  The subsequent clamp check is ineffective as it
only reduces ret_data_len.

Fix this by rejecting replies shorter than
sizeof(struct smb_snapshot_array) with -EIO. Note that the bound is set
to the 12-byte struct size rather than the 16-byte
MIN_SNAPSHOT_ARRAY_SIZE defined in MS-SMB2 3.3.5.15.1, because 12 bytes
is exactly what copy_to_user() attempts to read.

Fixes: e02789a53d71 ("smb3: enumerating snapshots was leaving part of the data off end")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>