libext2fs: add the EXT2FS_LINK_APPEND flag to ext2fs_link()

libext2fs: add the EXT2FS_LINK_APPEND flag to ext2fs_link()

Add a flag which only tries to add the new directory entry to the last
block in the directory.  This is helpful avoids mke2fs -d offering
from an O(n**2) performance bottleneck when adding a large number of
files to a directory.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>