btrfs: accessors: factor out split memcpy with two sources

btrfs: accessors: factor out split memcpy with two sources

The case of a reading the bytes from 2 folios needs two memcpy()s, the
compiler does not emit calls but two inline loops.

Factoring out the code makes some improvement (stack, code) and in the
future will provide an optimized implementation as well. (The analogical
version with two destinations is not done as it increases stack usage
but can be done if needed.)

The address of the second folio is reordered before the first memcpy,
which leads to an optimization reusing the vmemmap_base and
page_offset_base (implementing folio_address()).

Stack usage reduction:

  btrfs_get_32                                           -8 (32 -> 24)
  btrfs_get_64                                           -8 (32 -> 24)

Code size reduction:

     text    data     bss     dec     hex filename
  1454279  115665   16088 1586032  183370 pre/btrfs.ko
  1454229  115665   16088 1585982  18333e post/btrfs.ko

  DELTA: -50

As this is the last patch in this series, here's the overall diff
starting and including commit "btrfs: accessors: simplify folio bounds
checks":

Stack:

  btrfs_set_16                                          -72 (88 -> 16)
  btrfs_get_32                                          -56 (80 -> 24)
  btrfs_set_8                                           -72 (88 -> 16)
  btrfs_set_64                                          -64 (88 -> 24)
  btrfs_get_8                                           -72 (80 -> 8)
  btrfs_get_16                                          -64 (80 -> 16)
  btrfs_set_32                                          -64 (88 -> 24)
  btrfs_get_64                                          -56 (80 -> 24)

  NEW (48):
	  report_setget_bounds                           48
  LOST/NEW DELTA:      +48
  PRE/POST DELTA:     -472

Code:

     text    data     bss     dec     hex filename
  1456601  115665   16088 1588354  183c82 pre/btrfs.ko
  1454229  115665   16088 1585982  18333e post/btrfs.ko

  DELTA: -2372

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