xfs: fallback to vmalloc for large buffers in xfs_attrlist_by_handle

xfs: fallback to vmalloc for large buffers in xfs_attrlist_by_handle

Shamelessly copied from dchinner's:
ad650f5b xfs: fallback to vmalloc for large buffers in xfs_attrmulti_attr_get

xfsdump uses for a large buffer for extended attributes, which has a
kmalloc'd shadow buffer in the kernel. This can fail after the
system has been running for some time as it is a high order
allocation. Add a fallback to vmalloc so that it doesn't require
contiguous memory and so won't randomly fail while xfsdump is
running.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>