btrfs: accessors: set target address at initialization

The target address for the read/write can be simplified as it's the same
expression for the first folio. This improves the generated code as the
folio address does not have to be cached on stack.

Stack usage reduction:

  btrfs_set_32                                           -8 (32 -> 24)
  btrfs_set_64                                           -8 (32 -> 24)
  btrfs_get_16                                           -8 (24 -> 16)

Code size reduction:

     text    data     bss     dec     hex filename
  1454459  115665   16088 1586212  183424 pre/btrfs.ko
  1454279  115665   16088 1586032  183370 post/btrfs.ko

  DELTA: -180

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>