Unionfs ODF: Fix a lockdep warning on nfsd_readdir of exported unionfs mount

Unionfs ODF: Fix a lockdep warning on nfsd_readdir of exported unionfs mount

nfsd_readdir calls vfs_llseek and vfs_readdir. Following is the locking
order seen by lockdep:
	vfs_llseek: sb -> dentry -> inode
	vfs_readdir: inode -> sb -> dentry
Lockdep reports this as a possible circular locking order bug.

To fix this, we define our own unionfs_file_llseek_locked function,
which is essentially the vfs generic_file_llseek function, but with
the inode locking moved into the caller. This now changes the locking
order in vfs_llseek call to be: inode -> sb -> dentry, and satisfies
lockdep too.

Signed-off-by: Rachita Kothiyal <rachita@fsl.cs.sunysb.edu>