xfs: clean up symbolic link code

This series cleans up a few bits of the symbolic link code as needed for
future projects.  Online repair requires the ability to commit fixed
fork-based filesystem metadata such as directories, xattrs, and symbolic
links atomically, so we need to rearrange the symlink code before we
land the atomic extent swapping.

Accomplish this by moving the remote symlink target block code and
declarations to xfs_symlink_remote.[ch].

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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