arcstat: flush stdout / outfile after each line

arcstat: flush stdout / outfile after each line

Otherwise, if arcstat gets interrupted before the desired number of
iterations is reached, the output file will be empty (both if set via
'-o' or via shell redirection).

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Closes #6775