Make check is really slow, and a lot of it is in
compute_erofs_shared_xattrs() when doing hash comparisons.

Various improvements are don:
 * Estimate of the initial hash-table size to avoid resizes
 * Compare the xattr values before the xattr key names (as the kay names are often the same).
 * Don't divide by n_buckets each characted in hash_memory.

This drops a mkcomposefs --from-file run from 5 seconds to 1 second.

Signed-off-by: Alexander Larsson <[email protected]>

Each time a child is added we grow by 1, which causes a lot of
reallocs when adding children. Growing capacity by doubling saves us
about 10% of the time of a mkcomposefs --from-image run.

Signed-off-by: Alexander Larsson <[email protected]>