libmount: try absolute target before canonicalize

libmount: try absolute target before canonicalize

The path canonicalization is expensive and in many cases unwanted due
to problems with readlink() on unreachable NFS and automounters.

This patch add a possibility to search also by $(CWD)/<path> if the
<path> is relative to reduce number of situation when we convert the
path to the canonical absolute path.

The common use-case:

	# cd /some/long/path
	# umount ./mountpoint

old version:

15543: libmount:      TAB: [0x560a99a54230]: lookup TARGET: './test'
15543: libmount:    CACHE: [0x560a99a54290]: canonicalize path ./test
15543: libmount:    CACHE: [0x560a99a54290]: add entry [ 1] (path): /mnt/test: ./test
15543: libmount:      TAB: [0x560a99a54230]: lookup canonical TARGET: '/mnt/test'
15543: libmount:      CXT: [0x560a99a54050]: umount fs: /mnt/test

new version:

15597: libmount:      TAB: [0xabf230]: lookup TARGET: './test'
15597: libmount:      TAB: [0xabf230]: lookup absolute TARGET: '/mnt/test'
15597: libmount:      CXT: [0xabf050]: umount fs: /mnt/test

Signed-off-by: Karel Zak <kzak@redhat.com>