fs_mgr: fix clang static analyzer warning

Pointer from strdup is lost hence triggers mem leak warning from
clang, since ptr returned from basename may not point to start of
duplicated string any more.

Switch to use gnu version of basename which doesn't modify input
string so that strdup is no longer necessary.

Bug: 27126348
Test: compile

Change-Id: I937a68c01c223230932c932bffdd35da6503c3c4