e2fsck: skip quota update when interrupted

e2fsck: skip quota update when interrupted

There is a bug in how e2fsck handles being interrupted by CTRL-C.
If CTRL-C is pressed to kill e2fsck rather than e.g. kill -9, then
the interrupt handler sets E2F_FLAG_CANCEL in the context but doesn't
actually kill the process.  Instead, e2fsck_pass1() checks this flag
before processing the next inode.

If a filesystem is running in fix mode (e2fsck -fy) is interrupted,
and the quota feature is enabled, then the quota file will still be
written to disk even though the inode scan was not complete and the
quota information is totally inaccurate.  Even worse, if the Pass 1
inode and block scan was not finished, then the in-memory block
bitmaps (which are used for block allocation during e2fsck) are also
invalid, so any blocks allocated to the quota files may corrupt other
files if those blocks were actually used.

  e2fsck 1.42.13.wc3 (28-Aug-2015)
  Pass 1: Checking inodes, blocks, and sizes
  ^C[QUOTA WARNING] Usage inconsistent for ID 0:
      actual (6455296, 168) != expected (8568832, 231)
  [QUOTA WARNING] Usage inconsistent for ID 695:
      actual (614932320256, 63981) != expected (2102405386240, 176432)
  Update quota info for quota type 0? yes

  [QUOTA WARNING] Usage inconsistent for ID 0:
      actual (6455296, 168) != expected (8568832, 231)
  [QUOTA WARNING] Usage inconsistent for ID 538:
      actual (614932320256, 63981) != expected (2102405386240, 176432)
  Update quota info for quota type 1? yes

  myth-OST0001: e2fsck canceled.
  myth-OST0001: ***** FILE SYSTEM WAS MODIFIED *****

There may be a desire to flush out modified inodes and such that have
been repaired, so that restarting an interrupted e2fsck will make
progress, but the quota file update is plain wrong unless at least
pass1 has finished, and the journal recreation is also dangerous if
the block bitmaps have not been fully updated.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>