block/bdev.c: use the knowledge of inode/bdev coallocation

Here we know that bdevfs inodes are coallocated with struct block_device
and we can get to ->bd_inode value without any dereferencing.  Introduce
an inlined helper (static, *not* exported, purely internal for bdev.c)
that gets an associated inode by block_device - BD_INODE(bdev).

NOTE: leave it static; nobody outside of block/bdev.c has any business
playing with that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>