btrfs: rename check_async_write and let it return bool

The 'check_async_write' function is a helper used in
'btrfs_submit_metadata_bio' and it checks if asynchronous writing can be
used for metadata.

Make the function return bool and get rid of the local variable async in
btrfs_submit_metadata_bio storing the result of check_async_write's
tests.

As this is touching all function call sites, also rename it to
should_async_write as this is more in line with the naming we use.

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