logfs: rename functions starting with mtd_

logfs: rename functions starting with mtd_

We are going to re-work the MTD interface and change 'mtd->write()' to
'mtd_write()', 'mtd->read()' to 'mtd_read()' and so forth for all functions
in the 'struct mtd_info' structure.

However, logfs has its own 'mtd_read()', 'mtd_write()', etc functions
which collide with our changes. This patch renames these logfs functions
to 'logfs_mtd_read()', 'logfs_mtd_write()', etc.

Additionally, to make the 'fs/logfs/dev_mtd.c' file look consistent, rename
similarly all the other functions starting with 'mtd_'.

Cc: Jörn Engel <joern@logfs.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>