fs/adfs: map: rename adfs_map_free() to adfs_map_statfs()

adfs_map_free() is not obvious whether it is freeing the map or
returning the number of free blocks on the filesystem.  Rename it to
the more generic statfs() to make it clear that it's a statistic
function.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>