btrfs: simplify IS_ERR/PTR_ERR checks

IS_ERR(p) && PTR_ERR(p) == n is a weird way to spell p == ERR_PTR(n).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>