libext2fs: add the ext2fs_symlink() function

libext2fs: add the ext2fs_symlink() function

Creating symlinks is a complex affair when accounting for slowlinks.

Create a new function, ext2fs_symlink(), modeled after ext2fs_mkdir().
Like ext2fs_mkdir(), ext2fs_symlink() takes on the task of allocating a
new inode and block (for slowlinks), setting up sane default values in
the inode, copying the target path to either the inode (for fastlinks)
or to the first block (for slowlinks), and accounting for the inode and
block stats.  Disallow link targets longer than blocksize as the Linux
kernel prevents this.

It does not attempt to expand the parent directory, instead returning
EXT2_ET_DIR_NO_SPACE and leaving it to the caller to expand just as
ext2fs_mkdir() does.  Ideally, I think both of these functions should
make a single attempt to expand the directory.

[ Fixed a few bugs discovered when creating a test case for ext2fs_symlink() ]

Signed-off-by: Darren Hart <dvhart@infradead.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Andreas Dilger <adilger@dilger.ca>