fix(context/memory): incorrect remaining_src_span

The `AddrSpaceWrapper::r#move` is used by `mremap` to move, expand or
shrink existing mappings.

`src_span.count`: Size of old region in pages
`new_page_count`: Size of new region in pages

Fixes the following panic when running `cargo`:

```
KERNEL PANIC: panicked at src/context/memory.rs:1969:9:
assertion failed: self.info.can_extract(false)
  FP ffff80000cbbf2c0: PC ffffffff8004ced5
    FFFFFFFF8004CD50+0185
    kernel::panic::panic_handler_inner
  FP ffff80000cbbf3a0: PC ffffffff8004ac99
  FP ffff80000cbbf3b0: PC ffffffff800a34df
  FP ffff80000cbbf3e0: PC ffffffff800a34b4
  FP ffff80000cbbf430: PC ffffffff8001c029
    FFFFFFFF8001BF10+0119
    kernel::context::memory::Grant::extract
  FP ffff80000cbbf4a0: PC ffffffff800181ac
    FFFFFFFF80016D30+147C
    kernel::context::memory::AddrSpaceWrapper::move
  FP ffff80000cbbfd30: PC ffffffff80037c08
    FFFFFFFF80035660+25A8
    kernel::syscall::syscall
  FP ffff80000cbbfe90: PC ffffffff8008efbf
    FFFFFFFF8008EF10+00AF
    __inner_syscall_instruction
  FP ffff80000cbbff50: PC ffffffff80085d33
    FFFFFFFF80085CF0+0043
    kernel::arch::x86_64::interrupt::syscall::syscall_instruction
  0000000000004455: GUARD PAGE
CPU #2, CID 0xffffff7f80133690
NAME: /scheme/initfs/bin/redoxfs, DEBUG ID: 43
SYSCALL: mremap(0x2177F000, 0x8000, 0x0, 0xC7000, 0x60000)
HALT
```

```
mremap_flags = MremapFlags(
    0x0,
)
prot_flags = MapFlags(
    PROT_WRITE | PROT_READ,
)
```

Signed-off-by: Anhad Singh <andypython@protonmail.com>

src/context/memory.rs

@@ -405,7 +405,8 @@ impl AddrSpaceWrapper {
             )?);
         }
 
-        let mut remaining_src_span = PageSpan::new(src_span.base, new_page_count);
+        let mut remaining_src_span =
+            PageSpan::new(src_span.base, cmp::min(src_span.count, new_page_count));
 
         let to_remap = src_grants
             .conflicts(remaining_src_span)