Some fixes for pstore

1) Change from ->get_sb() to ->mount()
2) Use mount_single() instead of mount_nodev()
3) Pulled in ramfs_get_inode() & trimmed to what I need for pstore
4) Drop the ugly pstore_writefile() Just save data using kmalloc() and
   provide a pstore_file_read() that uses simple_read_from_buffer().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>