ext4: Add statx support

ext4: Add statx support

Return enhanced file attributes from the Ext4 filesystem.  This includes
the following:

 (1) The inode creation time (i_crtime) as stx_btime, setting STATX_BTIME.

 (2) Certain FS_xxx_FL flags are mapped to stx_attribute flags.

This requires that all ext4 inodes have a getattr call, not just some of
them, so to this end, split the ext4_getattr() function and only call part
of it where appropriate.

Example output:

	[root@andromeda ~]# touch foo
	[root@andromeda ~]# chattr +ai foo
	[root@andromeda ~]# /tmp/test-statx foo
	statx(foo) = 0
	results=fff
	  Size: 0               Blocks: 0          IO Block: 4096    regular file
	Device: 08:12           Inode: 2101950     Links: 1
	Access: (0644/-rw-r--r--)  Uid:     0   Gid:     0
	Access: 2016-02-11 17:08:29.031795451+0000
	Modify: 2016-02-11 17:08:29.031795451+0000
	Change: 2016-02-11 17:11:11.987790114+0000
	 Birth: 2016-02-11 17:08:29.031795451+0000
	Attributes: 0000000000000030 (-------- -------- -------- -------- -------- -------- -------- --ai----)

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>