e2fsprogs: fix device name parsing to resolve names containing '='

Currently in varisous e2fsprogs tools, most notably tune2fs and e2fsck
we will get the device name by passing the user provided string into
blkid_get_devname(). This library function however is primarily intended
for parsing "NAME=value" tokens. It will return the device matching the
specified token, NULL if nothing is found, or copy of the string if it's
not in "NAME=value" format.

However in case where we're passing in a file name that contains an
equal sign blkid_get_devname() will treat it as a token and will attempt
to find the device with the match. Likely finding nothing.

Fix it by checking existence of the file first and then attempt to call
blkid_get_devname(). In case of a collision, notify the user and
automatically prefer the one returned by blkid_get_devname(). Otherwise
return either the existing file, or NULL.

We do it this way to avoid some existing file in working directory (for
example LABEL=volume-name) masking an actual device containing the
matchin LABEL. User can specify full, or relative path (e.g.
./LABEL=volume-name) to make sure the file is used instead.

Link: https://lore.kernel.org/r/20220812130122.69468-1-lczerner@redhat.com
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Daniel Ng <danielng@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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