btrfs: set blocking_writers directly, no increment or decrement

The increment and decrement was inherited from previous version that
used atomics, switched in commit 06297d8c ("btrfs: switch
extent_buffer blocking_writers from atomic to int"). The only possible
values are 0 and 1 so we can set them directly.

The generated assembly (gcc 9.x) did the direct value assignment in
btrfs_set_lock_blocking_write (asm diff after change in 06297d8c):

     5d:   test   %eax,%eax
     5f:   je     62 <btrfs_set_lock_blocking_write+0x22>
     61:   retq

  -  62:   lock incl 0x44(%rdi)
  -  66:   add    $0x50,%rdi
  -  6a:   jmpq   6f <btrfs_set_lock_blocking_write+0x2f>

  +  62:   movl   $0x1,0x44(%rdi)
  +  69:   add    $0x50,%rdi
  +  6d:   jmpq   72 <btrfs_set_lock_blocking_write+0x32>

The part in btrfs_tree_unlock did a decrement because
BUG_ON(blockers > 1) is probably not a strong hint for the compiler, but
otherwise the output looks safe:

  - lock decl 0x44(%rdi)

  + sub    $0x1,%eax
  + mov    %eax,0x44(%rdi)

Signed-off-by: David Sterba <dsterba@suse.com>