xfs: refactor the inode fork memory allocation functions

xfs: refactor the inode fork memory allocation functions

Source kernel commit: 6c1c55ac3c0512262817a088e805d99aad4c0867

Hoist the code that allocates, frees, and reallocates if_broot into a
single xfs_iroot_krealloc function.  Eventually we're going to push
xfs_iroot_realloc into the btree ops structure to handle multiple
inode-rooted btrees, but first let's separate out the bits that should
stay in xfs_inode_fork.c.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>