VFS: Factor out part of vfs_setxattr so it can be called from the SELinux hook for...

VFS: Factor out part of vfs_setxattr so it can be called from the SELinux hook for inode_setsecctx.

This factors out the part of the vfs_setxattr function that performs the
setting of the xattr and its notification. This is needed so the SELinux
implementation of inode_setsecctx can handle the setting of the xattr while
maintaining the proper separation of layers.

Signed-off-by: David P. Quigley <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Signed-off-by: James Morris <[email protected]>