FIX: default kreadoff/kwriteoff must accept offset=0 for non-positioned I/O

The previous default only accepted u64::MAX as 'stream write' offset, but
sys_write/sys_read pass offset=0 for non-positioned I/O. This caused
ESPIPE on all writes to non-positioned FDs (Debug/stdout). Now both
u64::MAX and 0 are accepted as 'no offset' for the default implementation.
Schemes that override kwriteoff (AddrSpace) still handle offset=0 correctly.

src/scheme/mod.rs

@@ -551,7 +551,9 @@ pub trait KernelScheme: Send + Sync + 'static {
         stored_flags: u32,
         token: &mut CleanLockToken,
     ) -> Result<usize> {
-        if offset != u64::MAX {
+        // u64::MAX = explicit stream write; 0 = non-positioned write from sys_write/sys_read.
+        // Any other offset would imply seeking on a non-seekable scheme.
+        if offset != u64::MAX && offset != 0 {
             return Err(Error::new(ESPIPE));
         }
         self.kwrite(id, buf, flags, stored_flags, token)
@@ -565,7 +567,7 @@ pub trait KernelScheme: Send + Sync + 'static {
         stored_flags: u32,
         token: &mut CleanLockToken,
     ) -> Result<usize> {
-        if offset != u64::MAX {
+        if offset != u64::MAX && offset != 0 {
             return Err(Error::new(ESPIPE));
         }
         self.kread(id, buf, flags, stored_flags, token)

src/syscall/fs.rs

@@ -832,7 +832,7 @@ pub fn sys_read(fd: FileHandle, buf: UserSliceWo, token: &mut CleanLockToken) ->
             let offset = if desc.internal_flags.contains(InternalFlags::POSITIONED) {
                 desc.offset
             } else {
-                u64::MAX
+                0
             };
             Ok((
                 scheme.kreadoff(desc.number, buf, offset, desc.flags, desc.flags, token)?,
@@ -861,7 +861,7 @@ pub fn sys_write(fd: FileHandle, buf: UserSliceRo, token: &mut CleanLockToken) -
             let offset = if desc.internal_flags.contains(InternalFlags::POSITIONED) {
                 desc.offset
             } else {
-                u64::MAX
+                0
             };
             Ok((
                 scheme.kwriteoff(desc.number, buf, offset, desc.flags, desc.flags, token)?,