btrfs: don't use token set/get accessors for btrfs_item members

The token versions of set/get accessors will be removed, use the normal
helpers. The btrfs_item members use that interface the most but there
are no real benefits anymore.

This reduces stack consumption on x86_64 release config:

setup_items_for_insert                           -32 (144 -> 112)
__push_leaf_left                                 -32 (176 -> 144)
btrfs_extend_item                                -16 (104 -> 88)
copy_for_split                                   -32 (144 -> 112)
btrfs_del_items                                  -16 (144 -> 128)
btrfs_truncate_item                              -24 (152 -> 128)
__push_leaf_right                                -24 (168 -> 144)

and module size:

   text    data     bss     dec     hex filename
1463615  115665   16088 1595368  1857e8 pre/btrfs.ko
1463413  115665   16088 1595166  18571e post/btrfs.ko

DELTA: -202

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>