Move BCACHEFS_STATFS_MAGIC value to UAPI <linux/magic.h> under
BCACHEFS_SUPER_MAGIC definition (use common approach for name) and reuse the
definition in bcachefs_format.h BCACHEFS_STATFS_MAGIC.

There are other bcachefs magic definitions: BCACHE_MAGIC, BCHFS_MAGIC,
which use UUID_INIT() and are used only in libbcachefs. Therefore move
only BCACHEFS_STATFS_MAGIC value, which can be used outside of
libbcachefs for f_type field in struct statfs in statfs() or fstatfs().
Suggested-by: Su Yue <glass.su@suse.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Acked-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>