Bluetooth: Fix a redundant and problematic incoming MTU check

The L2CAP MTU for incoming data is verified differently depending on
the L2CAP mode, so the check is best performed in a mode-specific
context.  Checking the incoming MTU before HCI fragment reassembly is
a layer violation and assumes all bytes after the standard L2CAP
header are L2CAP data.

This approach causes issues with unsegmented ERTM or streaming mode
frames, where there are additional enhanced or extended headers before
the data payload and possible FCS bytes after the data payload.  A
valid frame could be as many as 10 bytes larger than the MTU.

Removing this code is the best fix, because the MTU is checked later
on for all L2CAP data frames (connectionless, basic, ERTM, and
streaming).  This also gets rid of outdated locking (socket instead of
l2cap_chan) and an extra lookup of the channel ID.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Reviewed-by: Ulisses Furquim <ulisses@profusion.mobi>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>