mm: Micro-optimize PID map reads for arm64 while retaining output format

Android and various applications in Android need to read PID map data in
order to work. Some processes can contain over 10,000 mappings, which
results in lots of time wasted on simply generating strings. This wasted
time adds up, especially in the case of Unity-based games, which utilize
the Boehm garbage collector. A game's main process typically has well
over 10,000 mappings due to the loaded textures, and the Boehm GC reads
PID maps several times a second. This results in over 100,000 map
entries being printed out per second, so micro-optimization here is
important. Before this commit, show_vma_header_prefix() would typically
take around 1000 ns to run on a Snapdragon 855; now it only takes about
50 ns to run, which is a 20x improvement.

The primary micro-optimizations here assume that there are no more than
40 bits in the virtual address space, hence the CONFIG_ARM64_VA_BITS
check. Arm64 uses a virtual address size of 39 bits, so this perfectly
covers it.

This also removes padding used to beautify PID map output to further
speed up reads and reduce the amount of bytes printed, and optimizes the
dentry path retrieval for file-backed mappings. Note, however, that the
trailing space at the end of the line for non-file-backed mappings
cannot be omitted, as it breaks some PID map parsers.

This still retains insignificant leading zeros from printed hex values
to maintain the current output format.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>