NFS: Save struct inode * inside nfs_commit_info to clarify usage of i_lock

Commit ea2cf228 created nfs_commit_info and saved &inode->i_lock inside
this NFS specific structure.  This obscures the usage of i_lock.
Instead, save struct inode * so later it's clear the spinlock taken is
i_lock.

Should be no functional change.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>