bitops: protect variables in set_mask_bits() macro

Unprotected naming of local variables within the set_mask_bits() can easily
lead to using the wrong scope.

Noticed this when "set_mask_bits(&foo->bar, 0, mask)" behaved as no-op.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 00a1a053 ("ext4: atomically set inode->i_flags in ext4_set_inode_flags()")
Cc: Theodore Ts'o <tytso@mit.edu>