debugfs: add symlink command

debugfs: add symlink command

Add support for symbolic links using a new symlink command.  Modeled
after the do_mkdir() command.

Testing demonstrates both fastlinks and slowlinks work correctly.
Very long target paths fail as the command parsing appears to truncate
the input to somewhere around 256 bytes.

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>