sys/gnu/vfs/ext2fs: Isolate Linux kernel code (just like other files do)

sys/gnu/vfs/ext2fs: Isolate Linux kernel code (just like other files do)

- dfly ext2 isolates code directly from Linux to separate files.
  This commit isolates two functions that are not isolated.

- Isolate functions ext2_check_descriptors() and ext2_statfs()
  to ext2fs/ext2_linux_super.c as these two are directly from
  fs/ext2/super.c (and comments say so).

- grep "Remy Card" to find out which files are from Linux kernel.
  # grep "Remy Card" sys/gnu/vfs/ext2fs -rIl
  sys/gnu/vfs/ext2fs/ext2_fs_sb.h
  sys/gnu/vfs/ext2fs/ext2_fs.h
  sys/gnu/vfs/ext2fs/ext2_linux_ialloc.c
  sys/gnu/vfs/ext2fs/ext2_linux_balloc.c
  sys/gnu/vfs/ext2fs/ext2_linux_super.c