octeontx2-af: debugfs: don't corrupt user memory

The user supplies the "count" value to say how big its read buffer is.
The rvu_dbg_lmtst_map_table_display() function does not take the "count"
into account but instead just copies the whole table, potentially
corrupting the user's data.

Introduce the "ret" variable to store how many bytes we can copy.  Also
I changed the type of "off" to size_t to make using min() simpler.

Fixes: 0daa55d0

 ("octeontx2-af: cn10k: debugfs for dumping LMTST map table")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211117073454.GD5237@kili


Signed-off-by: Jakub Kicinski <kuba@kernel.org>