Fix zfs_txg_timeout module parameter

Allow the zfs_txg_timeout variable to be dynamically tuned at run
time. By pulling it down out of the variable declaration it will
be evaluted each time through the loop.

The zfs_txg_timeout variable is now declared extern in a the common
sys/txg.h header rather than locally in dsl_scan.c. This prevents
potential type mismatches if the global variable needs to be used
elsewhere.

Move the module_param() code in to the same source file where
zfs_txg_timeout is declared. This is the most logical location.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>