um: Update UBD to use pread/pwrite family of functions

This decreases the number of syscalls per read/write by half.

Signed-off-by: Anton Ivanov <aivanov@brocade.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/file.c

@@ -264,6 +264,15 @@ int os_read_file(int fd, void *buf, int len)
 	return n;
 }
 
+int os_pread_file(int fd, void *buf, int len, unsigned long long offset)
+{
+	int n = pread(fd, buf, len, offset);
+
+	if (n < 0)
+		return -errno;
+	return n;
+}
+
 int os_write_file(int fd, const void *buf, int len)
 {
 	int n = write(fd, (void *) buf, len);
@@ -282,6 +291,16 @@ int os_sync_file(int fd)
 	return n;
 }
 
+int os_pwrite_file(int fd, const void *buf, int len, unsigned long long offset)
+{
+	int n = pwrite(fd, (void *) buf, len, offset);
+
+	if (n < 0)
+		return -errno;
+	return n;
+}
+
+
 int os_file_size(const char *file, unsigned long long *size_out)
 {
 	struct uml_stat buf;