quota: code cleanup for __dquot_alloc_space()

[ Upstream commit df15a2a59d0b29d86e17140b83ed231adaded12f ]

Replace (flags & DQUOT_SPACE_RESERVE) with
variable reserve.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Stable-dep-of: 179b8c97ebf6 ("quota: Fix rcu annotations of inode dquot pointers")
Signed-off-by: Sasha Levin <sashal@kernel.org>