i2c: core: check returned size of emulated smbus block read

If the i2c bus driver ignores the I2C_M_RECV_LEN flag (as some of
them do), it is possible for an I2C_SMBUS_BLOCK_DATA read issued
on some random device to return an arbitrary value in the first
byte (and nothing else).  When this happens, i2c_smbus_xfer_emulated()
will happily write past the end of the supplied data buffer, thus
causing Bad Things to happen.  To prevent this, check the size
before copying the data block and return an error if it is too large.

Fixes: 209d27c3

 ("i2c: Emulate SMBus block read over I2C")
Signed-off-by: Mans Rullgard <mans@mansr.com>
[wsa: use better errno]
Signed-off-by: Wolfram Sang <wsa@kernel.org>