ext4: code cleanup for ext4_statfs_project()

ext4: code cleanup for ext4_statfs_project()

[ Upstream commit a08fe66e4a0e12a3df982b28059f3a90e0f1b31e ]

Calling min_not_zero() to simplify complicated prjquota
limit comparison in ext4_statfs_project().

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Link: https://lore.kernel.org/r/20200210082445.2379-1-cgxu519@mykernel.net
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Stable-dep-of: f87d3af74193 ("ext4: don't over-report free space or inodes in statvfs")
Signed-off-by: Sasha Levin <sashal@kernel.org>