[PATCH 0/2] fs: two mount API fixes

[fujo]

Two mount API edge cases reported by shaos, both reproduced on
7.3.0-rc7 (58161ead2) with a small syscall probe run as /init under
QEMU with KASAN:

 - statmount(2) with STATMOUNT_MNT_UIDMAP on a mount that is not
   idmapped and a short buffer copied the reserved empty string at
   offset 512, one byte past the declared bufsize, and returned
   success.

 - listmount(2) and listns(2) with a U64_MAX cursor wrapped around to
   zero and restarted the enumeration at the first entry.

Probe output before:

  uidmap buf=100  ret=0   size=101 byte512=0
  lm cursor=MAX   ret=1   id0=2147483651
  ln cursor=MAX   ret=8   id0=1 id1=2

after:

  uidmap buf=100  ret=-75 (EOVERFLOW) byte512=170
  lm cursor=MAX   ret=-22 (EINVAL)
  ln cursor=MAX   ret=-22 (EINVAL)

The in-tree statmount_test (17/17), statmount_test_ns (2/2) and
listmount_test (2/2) pass with both patches applied.

Thanks,
fujo