libext2fs: check open(O_EXCL) first in ismounted.c

libext2fs: check open(O_EXCL) first in ismounted.c

Currently the ext2fs_check_mount_point() will use the open(O_EXCL) check
on linux after all the other checks are done. However it is not
necessary to check mntent if open(O_EXCL) succeeds because it means that
the device is not mounted.

Moreover the commit ea4d53b7 introduced a regression where a following
set of commands fails:

vgcreate mygroup /dev/sda
lvcreate -L 1G -n lvol0 mygroup
mkfs.ext4 /dev/mygroup/lvol0
mount /dev/mygroup/lvol0 /mnt
lvrename /dev/mygroup/lvol0 /dev/mygroup/lvol1
lvcreate -L 1G -n lvol0 mygroup
mkfs.ext4 /dev/mygroup/lvol0   <<<--- This fails

It fails because it thinks that /dev/mygroup/lvol0 is mounted because
the device name in /proc/mounts is not updated following the lvrename.

Move the open(O_EXCL) check before the mntent check and return
immediatelly if the device is not busy.

Fixes: ea4d53b7 ("libext2fs/ismounted.c: check device id in advance to skip false device names")
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Reported-by: Karel Zak <kzak@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>