btrfs-progs: refactor check_label()

btrfs-progs: refactor check_label()

Refactor check_label().

- Make it be static at first, this is a preparation step since we'll remove
btrfslabel.[c|h] and move those functions from there to utils.[c|h], we can
do pre-checking against the input label string with it.
- Fix the label length check up from BTRFS_LABEL_SIZE to BTRFS_LABEL_SIZE - 1.
- Kill the check of label contains an invalid character, see below commits for detail:
  79e0e445fc2365e47fc7f060d5a4445d37e184b8
  btrfs-progs: kill check for /'s in labels.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
CC: David Sterba <dsterba@suse.cz>
CC: Gene Czarcinski <gene@czarc.net>