cramfs: basic symlink support

cramfs: basic symlink support

Handle symlinks to files in the current directory. Other cases could be
handled with additional code, but this is a start.

Add explicit errors for absolute paths and links found in the middle of
a path (directories). Other cases like '..' or '.' will result with the
file not being found as when those path components are explicitly
provided.

Add a helper to decompress a null-terminated link name which is shared
with cramfs_list_inode.

Signed-off-by: Tyler Hall <tylerwhall@gmail.com>