[XFS] Refactor xfs_mountfs

Refactoring xfs_mountfs() to call sub-functions for logical chunks can
help save a bit of stack, and can make it easier to read this long
function.

The mount path is one of the longest common callchains, easily getting to
within a few bytes of the end of a 4k stack when over lvm, quotas are
enabled, and quotacheck must be done.

With this change on top of the other stack-related changes I've sent, I
can get xfs to survive a normal xfsqa run on 4k stacks over lvm.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:29834a
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>