xfs: split out the remote symlink handling

The remote symlink format definition and manipulation needs to be
shared with userspace, but the in-kernel interfaces do not. Split
the remote symlink format handling out into xfs_symlink_remote.[ch]
fo it can easily be shared with userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>