Remove incorrect assertion

Commit 85703f6 added a new ASSERT to zfs_write() as part of the
cleanup which isn't correct in the case where multiple processes
are concurrently extending a file. The zp->z_size is updated
atomically while holding a range lock on only a portion of the
file. Therefore, it's possible for the file size to increase
after a same check is performed earlier in the loop causing this
ASSERT to fail. The code itself handles this case correctly so
only the invalid ASSERT needs to be removed.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11235