NFSv4: include bitmap in nfsv4 get acl data

The NFSv4 bitmap size is unbounded: a server can return an arbitrary
sized bitmap in an FATTR4_WORD0_ACL request.  Replace using the
nfs4_fattr_bitmap_maxsz as a guess to the maximum bitmask returned by a server
with the inclusion of the bitmap (xdr length plus bitmasks) and the acl data
xdr length to the (cached) acl page data.

This is a general solution to commit e5012d1f "NFSv4.1: update
nfs4_fattr_bitmap_maxsz" and fixes hitting a BUG_ON in xdr_shrink_bufhead
when getting ACLs.

Fix a bug in decode_getacl that returned -EINVAL on ACLs > page when getxattr
was called with a NULL buffer, preventing ACL > PAGE_SIZE from being retrieved.

Cc: stable@kernel.org
Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/xdr.c

@@ -296,7 +296,7 @@ _copy_to_pages(struct page **pages, size_t pgbase, const char *p, size_t len)
  * Copies data into an arbitrary memory location from an array of pages
  * The copy is assumed to be non-overlapping.
  */
-static void
+void
 _copy_from_pages(char *p, struct page **pages, size_t pgbase, size_t len)
 {
 	struct page **pgfrom;
@@ -324,6 +324,7 @@ _copy_from_pages(char *p, struct page **pages, size_t pgbase, size_t len)
 
 	} while ((len -= copy) != 0);
 }
+EXPORT_SYMBOL_GPL(_copy_from_pages);
 
 /*
  * xdr_shrink_bufhead