Fix zpool-features(5) large_block inconsistency

Large_blocks feature activation was not consistent with man page,
which erroneously stated that the feature was active when the
recordsize was increased past the stock 128KB. It actually
becomes active when data is written to the dataset.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #6275
Closes #7093