[syserror] Split usermem package

Split usermem package to help remove syserror dependency in go_marshal.
New hostarch package contains code not dependent on syserror.

PiperOrigin-RevId: 365651233

pkg/sentry/fsimpl/host/BUILD

@@ -47,6 +47,7 @@ go_library(
         "//pkg/context",
         "//pkg/fdnotifier",
         "//pkg/fspath",
+        "//pkg/hostarch",
         "//pkg/iovec",
         "//pkg/log",
         "//pkg/marshal/primitive",

pkg/sentry/fsimpl/host/host.go

@@ -26,6 +26,7 @@ import (
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/fdnotifier"
 	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
 	"gvisor.dev/gvisor/pkg/sentry/hostfd"
@@ -431,8 +432,8 @@ func (i *inode) SetStat(ctx context.Context, fs *vfs.Filesystem, creds *auth.Cre
 		}
 		oldSize := uint64(hostStat.Size)
 		if s.Size < oldSize {
-			oldpgend, _ := usermem.PageRoundUp(oldSize)
-			newpgend, _ := usermem.PageRoundUp(s.Size)
+			oldpgend, _ := hostarch.PageRoundUp(oldSize)
+			newpgend, _ := hostarch.PageRoundUp(s.Size)
 			if oldpgend != newpgend {
 				i.CachedMappable.InvalidateRange(memmap.MappableRange{newpgend, oldpgend})
 			}

pkg/sentry/fsimpl/host/save_restore.go

@@ -21,9 +21,9 @@ import (
 
 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/fdnotifier"
+	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/safemem"
 	"gvisor.dev/gvisor/pkg/sentry/hostfd"
-	"gvisor.dev/gvisor/pkg/usermem"
 )
 
 // beforeSave is invoked by stateify.
@@ -38,7 +38,7 @@ func (i *inode) beforeSave() {
 		// EBADF from the read.
 		i.bufMu.Lock()
 		defer i.bufMu.Unlock()
-		var buf [usermem.PageSize]byte
+		var buf [hostarch.PageSize]byte
 		for {
 			n, err := hostfd.Preadv2(int32(i.hostFD), safemem.BlockSeqOf(safemem.BlockFromSafeSlice(buf[:])), -1 /* offset */, 0 /* flags */)
 			if n != 0 {