Complete MPU when allowed and under memory pressure

Fixes hang in a rather simple case:

rm mnt/file && \
fio -name=test -ioengine=libaio -direct=1 -iodepth=4 -bsrange=4k-512k \
    -rw=write -size=10G -filename=mnt/file && \
fio -name=test -ioengine=libaio -direct=1 -iodepth=4 -bsrange=4k-512k \
    -rw=randwrite -size=10G -filename=mnt/file

Previous fixes to "infinite loop" + GetHoles fix the same case but with
~15s pause between fio write and randwrite to allow GeeseFS to complete
MPU at least once

internal/file.go

@@ -769,7 +769,7 @@ func (inode *Inode) sendUpload() bool {
 
 	canComplete = canComplete && !inode.IsRangeLocked(0, inode.Attributes.Size, true)
 
-	if canComplete && (inode.fileHandles == 0 || inode.forceFlush) {
+	if canComplete && (inode.fileHandles == 0 || inode.forceFlush || atomic.LoadInt32(&inode.fs.wantFree) > 0) {
 		// Complete the multipart upload
 		inode.IsFlushing += inode.fs.flags.MaxParallelParts
 		atomic.AddInt64(&inode.fs.stats.flushes, 1)