NFS nfs_vm_page_mkwrite: Don't freeze me, Bro...

Prevent filesystem freezes while handling the write page fault.
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/file.c

@@ -569,6 +569,8 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 		filp, filp->f_mapping->host->i_ino,
 		(long long)page_offset(page));
 
+	sb_start_pagefault(inode->i_sb);
+
 	/* make sure the cache has finished storing the page */
 	nfs_fscache_wait_on_page_write(NFS_I(inode), page);
 
@@ -595,6 +597,7 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 out_unlock:
 	unlock_page(page);
 out:
+	sb_end_pagefault(inode->i_sb);
 	return ret;
 }