block/file-posix: update .help of BLOCK_OPT_PREALLOC option

Show 'falloc' among the allowed values of 'preallocation'
option, only when it is supported (if defined CONFIG_POSIX_FALLOCATE)

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190524075848.23781-3-sgarzare@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>