ext4: code cleanup for ext4_statfs_project()

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>