common/rc: improve block_size support for bcachefs

common/rc: improve block_size support for bcachefs

mkfs.bcachefs now supports option '--block_size' to allow
custom block_size.

Add the pattern to set def_blksz if MKFS_OPTIONS contains the
option in _scratch_mkfs_sized.
Also let mkfs.bcachefs decide blocksize if no option is given in
MKFS_OPTIONS or _scratch_mkfs_sized parameter.

Signed-off-by: Su Yue <glass.su@suse.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>