vfs_acl_xattr: avoid needlessly supplying a large buffer to getxattr()

vfs_acl_xattr: avoid needlessly supplying a large buffer to getxattr()

When obtaining the security descriptor via getxattr(), first try
optimistically to supply a buffer of 4K, and if that turns out
to be too small, determine the correct buffer size.

The previous behavior of falling back to a 64K buffer encountered
problem with Linux prior to version 3.6, due to pyisical memory
fragmentation. With those kernels, as long as the buffer is 8K or
smaller, getting the xattr is much less prone to failure due to
memory fragmentation.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12737

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Apr 18 04:41:16 CEST 2017 on sn-devel-144