use sendfile/splice implementation to perform DMA (#18411)

sendfile implementation to perform DMA on all platforms

Go stdlib already supports sendfile/splice implementations
for

- Linux
- Windows
- *BSD
- Solaris

Along with this change however O_DIRECT for reads() must be
removed as well since we need to use sendfile() implementation

The main reason to add O_DIRECT for reads was to reduce the
chances of page-cache causing OOMs for MinIO, however it would
seem that avoiding buffer copies from user-space to kernel space
this issue is not a problem anymore.

There is no Go based memory allocation required, and neither
the page-cache is referenced back to MinIO. This page-
cache reference is fully owned by kernel at this point, this
essentially should solve the problem of page-cache build up.

With this now we also support SG - when NIC supports Scatter/Gather
https://en.wikipedia.org/wiki/Gather/scatter_(vector_addressing)

internal/config/api/help.go

@@ -94,7 +94,7 @@ var (
 		},
 		config.HelpKV{
 			Key:         apiODirect,
-			Description: "set to enable or disable O_DIRECT for read and writes under special conditions. NOTE: do not disable O_DIRECT without prior testing" + defaultHelpPostfix(apiODirect),
+			Description: "set to enable or disable O_DIRECT for writes under special conditions. NOTE: do not disable O_DIRECT without prior testing" + defaultHelpPostfix(apiODirect),
 			Optional:    true,
 			Type:        "boolean",
 		},