ext4: use XATTR_CREATE in ext4_initxattrs()

I hit ENOSPC error when creating new file in a newly created ext4
with ea_inode feature enabled, if selinux is enabled and ext4 is
mounted without any selinux context. e.g.

  mkfs -t ext4 -O ea_inode -F /dev/sda5
  mount /dev/sda5 /mnt/ext4
  touch /mnt/ext4/testfile  # got ENOSPC here

It turns out that we run out of journal credits in
ext4_xattr_set_handle() when creating new selinux label for the
newly created inode.

This is because that in __ext4_new_inode() we use
__ext4_xattr_set_credits() to calculate the reserved credits for new
xattr, with the 'is_create' argument being true, which implies less
credits in the ea_inode case. But we calculate the required credits
in ext4_xattr_set_handle() with 'is_create' being false, which means
we need more credits if ea_inode feature is enabled. So we don't
have enough credits and error out with ENOSPC.

Fix it by simply calling ext4_xattr_set_handle() with XATTR_CREATE
flag in ext4_initxattrs(), so we end up with requiring less credits
than reserved. The semantic of XATTR_CREATE is "Perform a pure
create, which fails if the named attribute exists already." (from
setxattr(2)), which is fine in this case, because we only call
ext4_initxattrs() on newly created inode.

Fixes: af65207c ("ext4: fix __ext4_new_inode() journal credits calculation")
Cc: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>