Correctly handle return value of libc::mmap in `fs_calls.rs`

[qianxichen233]

The function libc::mmap returns a 64-bit address, which is directly used by several memory-related functions (e.g., shmat, mmap, etc.). However, when libc::mmap fails, its return value is not properly distinguished from a valid address.

Currently, the failure return value is treated as a regular u64 address. Since libc::mmap returns an error code in the same value space, this can lead to incorrect handling (e.g., interpreting an error code as a valid 64-bit address), which causes incorrect behavior or crashes.

We can adopt the same approach used in glibc/lind_syscall/lind_syscall.c to treat any return value that is not page-aligned as errno.