perf optimization on filestat in file.c

[mallicksm]

Summary:
Optimize stati() by replacing sleeplock (ilock()) with a spinlock for fast metadata access, avoiding unnecessary context switches.

Description:
Currently, stati() uses ilock(), which is a sleeplock, to prevent concurrent modifications when copying inode metadata. However, stati() is a quick read-only operation that only copies a few fields from struct inode to struct stat. Using a sleeplock is overkill and causes unnecessary thread rescheduling if the lock is already held.

Proposed Fix:
• Add a spinlock (inode_lock) to struct inode.
• Use this spinlock in stati() instead of ilock(), ensuring atomicity without costly context switches.
• Keep ilock() for long-duration operations like readi() and writei() that require sleeping.