nilfs-resize: add macro to print message when in verbose mode

Add and apply the verbose_msg() macro that prints the argument's
message when global variable "verbose" is true.

Also, shorten the name of the message output function "myprintf" for
coexistence with a progress bar to "msg".

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>

Author: konis