btrfs: rename set_extent_bit() to include a btrfs prefix

btrfs: rename set_extent_bit() to include a btrfs prefix

This is an exported function so it should have a 'btrfs_' prefix by
convention, to make it clear it's btrfs specific and to avoid collisions
with functions from elsewhere in the kernel.

So rename it to btrfs_set_extent_bit().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>