Merge FileMem.usage in IncRef

Per the doc, usage must be kept maximally merged. Beyond that, it is simply a
good idea to keep fragmentation in usage to a minimum.

The glibc malloc allocator allocates one page at a time, potentially causing
lots of fragmentation. However, those pages are likely to have the same number
of references, often making it possible to merge ranges.

PiperOrigin-RevId: 204960339
Change-Id: I03a050cf771c29a4f05b36eaf75b1a09c9465e14

pkg/sentry/fs/proc/README.md

@@ -11,6 +11,7 @@ inconsistency, please file a bug.
 
 The following files are implemented:
 
+
 | File /proc/                 | Content                                               |
 | :------------------------   | :---------------------------------------------------- |
 | [cpuinfo](#cpuinfo)         | Info about the CPU                                    |
@@ -22,6 +23,7 @@ The following files are implemented:
 | [uptime](#uptime)           | Wall clock since boot, combined idle time of all cpus |
 | [version](#version)         | Kernel version                                        |
 
+
 ### cpuinfo
 
 ```bash

pkg/sentry/platform/filemem/filemem.go

@@ -547,6 +547,8 @@ func (f *FileMem) IncRef(fr platform.FileRange) {
 	if gap.Ok() {
 		panic(fmt.Sprintf("IncRef(%v): attempted to IncRef on unallocated pages %v:\n%v", fr, gap.Range(), &f.usage))
 	}
+
+	f.usage.MergeAdjacent(fr)
 }
 
 // DecRef implements platform.File.DecRef.